<PAGE>   1
                UNITED STATES SECURITIES AND EXCHANGE COMMISSION

                              Washington, DC 20549
                                  FORM 10-K/A

[X]     Annual Report Pursuant to Section 13 or 15(d) of the Securities Exchange
        Act of 1934 for the Fiscal Year ended December 31, 1999

                                       OR

[ ]     Transition Report Pursuant to Section 13 or 15(d) of the Securities
        Exchange Act of 1934 Commission file number 0-6272

                                   DATUM INC.
             (Exact name of Registrant as specified in its charter)

               Delaware                                   95-2512237
     (State or other jurisdiction                      (I.R.S. Employer
     of incorporation or organization)                 Identification No.)

                    9975 Toledo Way, Irvine, California 92618
                    (Address of principal executive offices)
       Registrant's telephone number, including area code: (949) 598-7500

           Securities registered pursuant to Section 12(b) of the Act:

    Title of each class               Name of each exchange on which registered
    -------------------               -----------------------------------------
          None                                          None

           Securities registered pursuant to Section 12(g) of the Act:

                                  COMMON STOCK
                                  ------------
                                (Title of Class)

                           ---------------------------

Indicate by check mark whether the Registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during
the preceding 12 months (or for such shorter period that the Registrant was
required to file such reports), and (2) has been subject to such filing
requirements for the past 90 days.

Yes   [X]      No    [ ]

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405
of Regulation S-K is not contained herein, and will not be contained, to the
best of Registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. [X]

The aggregate market value of the voting stock held by non-affiliates of the
Registrant, based upon the closing sales price of the Common Stock as of March
20, 2000, was approximately $145,209,957

The number of outstanding shares of the Registrant's Common Stock as of March
20, 2000 was 5,897,138.

                       DOCUMENTS INCORPORATED BY REFERENCE

Portions of Registrant's Proxy Statement for the Annual Meeting of Stockholders
to be held on June 8, 2000 (to be filed with the Commission within 120 days of
December 31, 1999): Part III, Items 10-13.

                               Page 1 of ___ Pages
     Exhibit Index is Located on Sequential Numbered Page __ of this Report.

<PAGE>   2

                                   SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange
Act of 1934, the Registrant has duly caused this Amendment to Annual Report on
Form 10-K to be signed on its behalf by the undersigned, thereunto duly
authorized, at Irvine, California this 25th day of April, 2000.

                                            DATUM INC.

                                            By   /s/ Erik H. van der Kaay
                                                --------------------------------
                                                 Erik H. van der Kaay
                                                 President and Director


                                       2
<PAGE>   3
                                   DATUM INC.

                            FORM 10-K - ITEM 14(a)(3)

                  EXECUTIVE COMPENSATION PLANS AND ARRANGEMENTS

<TABLE>
<S>                 <C>
10.4                1984 Stock Option Plan, as amended to date (incorporated by
                    reference to Registrant's Registration Statements on Form
                    S-8 Registration numbers 2-96564, 33-10035 and 33-41709).

10.10               Form of Indemnification Agreement dated May 27, 1987 as
                    entered into with certain directors and officers of
                    Registrant (incorporated by reference to same numbered
                    exhibit to Registrant's Annual Report on Form 10-K for the
                    year ended December 31, 1991).

10.21               Consulting Agreement dated October 9, 1992 with Louis B.
                    Horwitz (incorporated by reference to same numbered exhibit
                    to Registrant's Annual Report on Form 10-K for the year
                    ended December 31, 1992).

10.21.1             First Amendment to Consulting Agreement, dated as of March
                    1, 1996, between Louis B. Horwitz and the Registrant
                    (incorporated by reference to the same numbered exhibit to
                    Registrant's Quarterly Report on Form 10-Q for the quarter
                    ended September 30, 1996).

10.29               1994 Stock Incentive Plan (incorporated by reference to
                    Registrant's Registration Statement on Form S-8,
                    Registration No. 33-79772).

10.29.1             Amendment to 1994 Stock Incentive Plan, effective March 16,
                    1995 (incorporated by reference to the same numbered exhibit
                    to Registrant's Form 10-K for the year ended December 31,
                    1994).

10.29.2             Second Amendment to 1994 Stock Incentive Plan, effective
                    June 5, 1997 (incorporated by reference to Registrant's
                    Registration Statement on Form S-8, Registration No.
                    33-79772).

10.41               Employee Stock Purchase Plan (incorporated by reference to
                    registrant proxy statement for its Annual Meeting of
                    Stockholders on June 5, 1997, filed with the commission on
                    May 1, 1997).

10.42               Employment Agreement, dated March 27, 1998, between the
                    Company and Erik H. van der Kaay (incorporated by reference
                    to the same numbered exhibit to the Registrant's Quarterly
                    Report on Form 10-Q for the quarter ended March 31, 1998).

10.43               Non-qualified stock option agreement, dated April 6, 1998,
                    between the Company and Erik H. van der Kaay (incorporated
                    by reference to the same numbered exhibit to the
                    Registrant's Quarterly Report on Form 10-Q for the quarter
                    ended June 30, 1998).

10.44               Restricted stock grant agreement, dated April 6, 1998,
                    between the Company and Erik H. van der Kaay (incorporated
                    by reference to the same numbered exhibit to the
                    Registrant's Quarterly Report on Form 10-Q for the quarter
                    ended June 30, 1998)

10.46               Employment Agreement, dated July 29, 1999, between the
                    Company and Thomas Mark Hastings.*

10.47               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and Erik van der Kaay.

10.48               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and David A. Young.

10.49               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and Paul E. Baia.

10.50               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and Michael J. Patrick.

10.51               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and John R. Rice.

10.52               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and Raymond L. Waguespack.
</TABLE>

------------
* Previously filed

<PAGE>   4

                                  EXHIBIT INDEX


<TABLE>
<CAPTION>
                                                                                    SEQUENTIALLY
EXHIBIT                                                                               NUMBERED
NUMBER              DESCRIPTION                                                         PAGE
------              -----------                                                     ------------
<S>                 <C>                                                             <C>
 2.2                Agreement and Plan of Merger Agreement, dated July 29, 1999,
                    among the Registrant, Digital Delivery, Inc., certain
                    stockholders of Digital Delivery and Datum Acquisition Sub,
                    Inc. (incorporated by reference to the Registrant's Current
                    Report on Form 8-K, dated July 29, 1999, as amended)

 3.1                Certificate of Incorporation of Datum Inc., a Delaware
                    corporation, as amended to date (incorporated by reference
                    to the same numbered exhibit on Form 10-Q for the quarter
                    ended June 30, 1996).                                                --

 3.2                Bylaws of Datum Inc. as amended to date (incorporated by
                    reference to the exhibit 1 on Form 8-K dated November 17,
                    1999).                                                               --

 4.2                Rights Agreement, dated as of November 8, 1999, between the
                    Registrant and ChaseMellon Shareholder Services, L.L.C.
                    (incorporated by reference to Exhibit 1 to the Company's
                    Registration Statement on Form 8-A, Registration No.
                    000-06272)

10.4                1984 Stock Option Plan, as amended to date (incorporated by
                    reference to Registrant's Registration Statements on Form
                    S-8, Registration No.'s 2-96564, 33-10035 and 33-41709).             --

10.10               Form of Indemnification Agreement dated May 27, 1987 as
                    entered into with certain directors and officers of
                    Registrant (incorporated by reference to same numbered
                    exhibit to Registrant's Annual Report on Form 10-K for the
                    year ended December 31, 1991).                                       --

10.19               Savings and Retirement Plan, as amended to date
                    (incorporated by reference to same numbered exhibit to
                    Registrant's Annual Report on Form 10-K for the year ended
                    December 31, 1991).                                                  --

10.21               Consulting Agreement dated October 9, 1992 with Louis B.
                    Horwitz (incorporated by reference to same numbered exhibit
                    to Registrant's Annual Report on Form 10-K for the year
                    ended December 31, 1992).                                            --

10.21.1             First Amendment to Consulting Agreement, dated as of March
                    1, 1996, between Louis B. Horwitz and the Registrant
                    (incorporated by reference to the same numbered exhibit to
                    Registrant's Quarterly Report on Form 10-Q for the quarter
                    ended September 30, 1996).                                           --

10.29               1994 Stock Incentive Plan (incorporated by reference to
                    Registrant's Registration Statement on Form S-8,
                    Registration No. 33-79772).                                          --

10.29.1             Amendment to 1994 Stock Incentive Plan, effective March 17,
                    1995. (incorporated by reference to the same numbered
                    exhibit on Form 10-K for the year ended December 31, 1994).          --
</TABLE>

<PAGE>   5

                                  EXHIBIT INDEX


<TABLE>
<CAPTION>
                                                                                    SEQUENTIALLY
EXHIBIT                                                                               NUMBERED
NUMBER              DESCRIPTION                                                         PAGE
------              -----------                                                     ------------
<S>                 <C>                                                             <C>
10.29.2             Second Amendment to 1994 Stock Incentive Plan, effective
                    June 5, 1997 (incorporated by reference to Registrant's
                    Registration Statement on Form S-8, Registration No.
                    33-79772).                                                           --

10.30.5             Amended and Restated Credit Agreement dated as of September
                    27, 1996, by and between the Registrant and Wells Fargo
                    Bank, N.A. (incorporated by reference to the same numbered
                    exhibit to the Registrant's Quarterly Report on Form 10-Q
                    for the quarter ended September 30, 1996).                           --

10.32               Lease Agreement dated September 15, 1986 by and between The
                    Irvine Company and Efratom Division, Ball Corporation, for
                    Efratom Time and Frequency Products, Inc.'s facility at 3
                    Parker, Irvine, California. (incorporated by reference to
                    the same numbered exhibit on Form 10-K for the year ended
                    December 31, 1994).                                                  --

10.32.1             First Amendment to Lease dated March 15, 1995 by and between
                    The Irvine Company and Efratom Division, Ball Corporation
                    for Lease Agreement dated September 15, 1986 (Exhibit 10.32)
                    (incorporated by reference to the same numbered exhibit on
                    Form 10-K for the year ended December 31, 1994).                     --

10.32.2             Amendment to Leases dated May 11, 1995 between the Irvine
                    Company and the Registrant (incorporated by reference to the
                    same numbered exhibit on Form 10-Q for the quarter ended
                    June 30, 1995).                                                      --

10.32.4             Second Amendment to Lease dated May 11, 1995 for 3 Parker
                    (incorporated by reference to the same numbered exhibit on
                    Form 10-Q for the quarter ended June 30, 1995).                      --

10.34               Industrial Lease dated May 11, 1995 between the Irvine
                    Company and the Registrant (incorporated by reference to the
                    same numbered exhibit to the Registrant's Form 10-Q for the
                    quarter ended June 30, 1995).                                        --

10.35               Lease Agreement dated January 4, 1996, by and between Berg &
                    Berg Developers and the Registrant relating to Registrant's
                    Facility at 6781 Via Del Oro, San Jose, California
                    (incorporated by reference to the same numbered exhibit to
                    the Registrant's Form 10-K for the year ended December 31,
                    1995.                                                                --

10.36               Note and Warrant Purchase Agreement, dated as of September
                    27, 1996, by and between The Prudential Insurance Company of
                    America and the Registrant (incorporated by reference to the
                    same numbered exhibit to the Registrant's Quarterly Report
                    on Form 10-Q for the quarter ended September 30, 1996.               --
</TABLE>

<PAGE>   6

                                  EXHIBIT INDEX

<TABLE>
<CAPTION>
                                                                                    SEQUENTIALLY
EXHIBIT                                                                               NUMBERED
NUMBER              DESCRIPTION                                                         PAGE
------              -----------                                                     ------------
<S>                 <C>                                                             <C>
10.37               Common Stock Purchase Warrant, dated September 27, 1996
                    (incorporated by reference to the same numbered exhibit to
                    the Registrant's Quarterly Report on Form 10-Q for the
                    quarter ended September 30, 1996).                                   --

10.38               Series A Senior Secured Notes, dated September 27, 1996, in
                    favor of The Prudential Insurance Company of America
                    (incorporated by reference to the same numbered exhibit to
                    the Registrant's Quarterly Report on Form 10-Q for the
                    quarter ended September 30, 1996).                                   --

10.39               Series B Senior Secured Notes, dated September 27, 1996, in
                    favor of The Prudential Insurance Company of America
                    (incorporated by reference to the same numbered exhibit to
                    the Registrant's Quarterly Report on Form 10-Q for the
                    quarter ended September 30, 1996).                                   --

10.41               Employee Stock Purchase Plan (incorporated by reference to
                    registrant proxy statement for its Annual Meeting of
                    Stockholders on June 5, 1997, filed with the commission on
                    May 1, 1997).                                                        --

10.42               Employee Agreement, dated March 27, 1998, between the
                    Company and Erik H. van der Kaay (incorporated by reference
                    to the same numbered exhibit to the Registrant's Quarterly
                    Report on Form 10-Q for the quarter ended March 31, 1998).           --

10.43               Non-qualified stock option agreement, dated April 6, 1998,
                    between the Company and Erik H. van der Kaay (incorporated
                    by reference to the same numbered exhibit to the
                    Registrant's Quarterly Report on Form 10-Q for the quarter
                    ended June 30, 1998.                                                 --

10.44               Restricted stock grant agreement, dated April 6, 1998,
                    between the Company and Erik H. van der Kaay (incorporated
                    by reference to the same numbered exhibit to the
                    Registrant's Quarterly Report on Form 10-Q for the quarter
                    ended June 30, 1998.                                                 --

10.45               Agreement with Lucent Technologies Inc., signed July 2,
                    1998. (incorporated by reference to the same numbered
                    exhibit to the Registrant's Annual Report on Form 10-K for
                    the year ended December 31, 1998. Portions of this Exhibit
                    are omitted and were filed separately with the Secretary of
                    the Commission pursuant to the Company's application
                    requesting confidential treatment under Rule 406 of the
                    Securities Act of 1933.)

10.46               Employment Agreement, dated July 29, 1999, between the
                    Company and Thomas Mark Hastings.*                                   --

10.47               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and Erik van der Kaay.                    --

10.48               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and David A. Young.                       --

10.49               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and Paul E. Baia.                         --

10.50               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and Michael J. Patrick.                   --

10.51               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and John R. Rice.                         --

10.52               Severance Compensation Agreement, dated October 29, 1999, by
                    and between the Registrant and Raymond L. Waguespack.                --

21                  List of Subsidiaries*                                                --

23.1                Consent of Independent Accountants*                                  --

23.2                Consent of Independent Accountants*                                  --

27.4                Financial Data Schedule Year Ended 1999*                             --
</TABLE>

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* Previously filed