If you do not care about fractions of seconds, convert the $H values to count of seconds .. $zdt(datetime,-2) .. and then compare the integers:

set date1 = $zdth("2016-01-02 00:15",3)

set date2 = $zdth("2016-01-02 01:00",3)

zw date1,date2

w $zdt(date1,-2)>$zdt(date2,-2)

If you do care about the seconds fractions you can use the follows operator with some of the external formats:

s d1=$zts h .001 s d2=$zts

zw d1,d2
d1="64180,33196.069"
d2="64180,33196.07"

w $zdt(d1,3,1,6)]$zdt(d2,3,1,6)

0

w $zdt(d2,3,1,6)]$zdt(d1,3,1,6)

1

/// note: comparing the count of seconds would consider the d1 and d2 equal.