<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, 1998

                                       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
17, 1999, was approximately $36,920,411.

The number of outstanding shares of the Registrant's Common Stock as of March
17, 1999 was 5,528,266.

                       DOCUMENTS INCORPORATED BY REFERENCE

Portions of Registrant's Proxy Statement for the Annual Meeting of Stockholders
to be held on June 10, 1999 (to be filed with the Commission within 120 days of
December 31, 1998): 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 Annual Report on Form 10-K/A to
be signed on its behalf by the undersigned, thereunto duly authorized, at
Irvine, California this 29th day of March, 1999.

                                               DATUM INC.

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

                                POWER OF ATTORNEY

The undersigned directors and officers of Datum Inc. constitutes and appoints as
their true and lawful attorney and agent with power of substitution, to do any
and all acts and things in our name and behalf in our capacities as directors
and officers and to execute any and all instruments for us and in our names in
the capacities indicated below, which said attorney and agent, may deem
necessary or advisable to enable said corporation to comply with the Securities
Exchange Act of 1934, as amended and any rules, regulations and requirements of
the Securities and Exchange Commission, in connection with this Annual Report on
Form 10-K/A, including specifically but without limitation, power and authority
to sign for us or any of us in our names in the capacities indicated below, any
and all amendments (including post-effective amendments) hereto: and we do
hereby ratify and confirm all that said attorney and agent, shall do or cause to
be done by virtue hereof.

Pursuant to the requirements of the Securities Exchange Act of 1934, this report
has been signed below by the following persons on behalf of the registrant and
in the capacities and on the date indicated.

<TABLE>
<S>                                     <C>                                               <C>

/s/ Erik H. van der Kaay                President and Director                            March 29, 1999
------------------------------------    (Principal Executive Officer)
Erik H. van der Kaay                    

/s/ David A. Young                      Chief Financial Officer                           March 29, 1999
-----------------------------------     (Principal Financial and Accounting Officer)
David A. Young                     

/s/ Louis B. Horwitz                    Director and Chairman                             March 29, 1999
-----------------------------------
Louis B. Horwitz

 /s/ G. Tilton Gardner                  Director                                          March 29, 1999
-----------------------------------
G. Tilton Gardner

 /s/ Dan L. McGurk                      Director                                          March 29, 1999
-----------------------------------
Dan L. McGurk

 /s/ Edward A. Money                    Director                                          March 29, 1999
-----------------------------------
Edward A. Money

 /s/ Michael M. Mann                    Director                                          March 29, 1999
-----------------------------------
Michael M. Mann

 /s/ R. David Hoover                    Director                                          March 29, 1999
-----------------------------------
R. David Hoover
</TABLE>


                                       21
<PAGE>   3

                          DATUM INC. AND SUBSIDIARIES

                           CONSOLIDATED BALANCE SHEET

                        (IN THOUSANDS, EXCEPT SHARE DATA)

<TABLE>
<CAPTION>
                                                                                     DECEMBER 31,
                                                                                ----------------------
                                                                                  1998          1997
                                                                                --------      --------
<S>                                                                             <C>           <C>     
ASSETS
Current assets:
  Cash and cash equivalents                                                     $ 10,307      $  5,819
  Accounts receivable, less allowance for doubtful accounts of $153 and $71       19,327        15,043
  Inventories                                                                     24,555        31,219
  Prepaid expenses                                                                   479           363
  Deferred income taxes                                                            3,056         2,648
  Income tax refund receivable                                                     1,190         1,321
                                                                                --------      --------
          Total current assets                                                    58,914        56,413
Land, buildings and equipment, net                                                16,048        16,791
Excess of purchase price over net assets acquired, net of accumulated
  amortization of $3,845 and $2,951                                               11,231        12,126
Other assets                                                                         727           416
                                                                                --------      --------
                                                                                $ 86,920      $ 85,746
                                                                                ========      ========
LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
  Accounts payable                                                              $  4,241      $  3,343
  Accrued salaries and wages                                                       2,485         3,298
  Accrued warranty                                                                 1,498           990
  Other accrued expenses                                                           1,287         1,277
  Income taxes payable                                                               289            --
  Current portion of long-term debt                                                3,025            23
                                                                                --------      --------
          Total current liabilities                                               12,825         8,931
                                                                                --------      --------
Long-term debt                                                                    14,533        17,418
                                                                                --------      --------
Post-retirement benefits                                                             818           602
                                                                                --------      --------
Other long-term liabilities                                                          144           128
                                                                                --------      --------
Deferred income taxes                                                              1,622         1,823
                                                                                --------      --------
Stockholders' equity:
  Preferred stock, par value $.25 per share
     Authorized - 1,000,000 shares
     Issued - none
  Common stock, par value $.25 per share
     Authorized - 10,000,000 shares
     Issued - 5,505,843 shares in 1998 and 5,332,860 shares in 1997                1,376         1,333
  Additional paid-in capital                                                      44,941        43,231
  Retained earnings                                                               11,328        12,785
  Unamortized stock compensation                                                    (368)           --
  Accumulated other comprehensive income                                            (299)         (505)
                                                                                --------      --------
          Total stockholders' equity                                              56,978        56,844
                                                                                --------      --------
Commitments (Notes C and H)                                                     $ 86,920      $ 85,746
                                                                                ========      ========
</TABLE>


                 See Notes to Consolidated Financial Statements


                                      F-2