original checkin message:

    DateTime.__init__() and DateTime.__str__():  When a fractional
    number of seconds is passed in or derived, round it to the
    nearest millisecond.  Also changed str() to produce no more than
    three digits after the second's decimal point (if any).

    This ensures that DateTime->string->DateTime loses no information.
    Before this change, platform rounding errors in float<->string
    conversion could lose information in both conversion steps, so a
    DateTime object converted to string and back again didn't always
    reproduce the input object's value.  That's bad on its own, and
    testDateTime.py's testConstructor3() failed about 1% of the time
    on Linux as a result.  It doesn't fail after this patch.

    CAUTION:  Note that DateTime objects have at finest millisecond
    resolution now, even if the platform time.time() happens to have
    resolution finer than that.

Note that he also had to fiddle with the function that returned system
time, so he's got a wrapper around time.time() that rounds to 3 places
the float returned by system time.   All the testing I've done
indicates that this intermittent fixes the comparison failures.