Sybase dataadd time where time added is the difference of 2 times -

i have sybase database , create new bigdatetimefield adding time current bigdatetimefield

for example

i have date1 field =   8/31/2015 2:23:49.529000 pm     have date2 field  =    8/31/2015 7:23:49.529000 have maindate field =  8/31/2015 2:24:46.112000 pm 

i make new field maindate field minus difference in time between date1 field , date2 field

so in case new filed 8/31/2015 2:24:46.112000 pm - (8/31/2015 2:23:49.529000 pm - 8/31/2015 7:23:49.529000 am)

any idea how in sybase?

select new_dt = dateadd(ss, datediff(ss,date1,date2),maindate)  my_table 

that's accurate seconds though. can use millisecond or microsecond.