Max min bugfix

I found the following behavior when calling timestamp-minimize-part and timestamp-maximize-part 

LT> (timestamp-minimize-part @2014-11-02T16:20:44.112318-06:00 :hour)
@2014-11-02T01:00:00.000000-05:00
LT> (timestamp-maximize-part @2014-11-02T01:20:44.112318-05:00 :hour)
@2014-11-02T22:59:59.999999-06:00
LT> (timestamp-maximize-part @2015-03-08T01:20:44.112318-06:00 :hour)
@2015-03-09T00:59:59.999999-05:00
LT> (timestamp-minimize-part @2015-03-08T16:20:44.112318-06:00 :hour)
@2015-03-07T23:00:00.000000-06:00

I believe this is unexpected behavior.  This change results in the following:

LT> (timestamp-minimize-part @2014-11-02T16:20:44.112318-06:00 :hour)
@2014-11-02T00:00:00.000000-05:00
LT> (timestamp-maximize-part @2014-11-02T01:20:44.112318-05:00 :hour)
@2014-11-02T23:59:59.999999-06:00
LT> (timestamp-maximize-part @2015-03-08T01:20:44.112318-06:00 :hour)
@2015-03-08T23:59:59.999999-05:00
LT> (timestamp-minimize-part @2015-03-08T16:20:44.112318-06:00 :hour)
@2015-03-08T00:00:00.000000-06:00