Hi,
I am not sure but might be the time zone is causing an issue.
I am not sure whetehr below logic could be useful or not but you can give a try on development system and perform all test cases.
LOOP AT s_s_if-t_select INTO l_s_select WHERE fieldnm = 'ZZTMSTMP'.
tstamp = l_s_select-low.
timezone = 'EST'.
CONVERT TIME STAMP tstamp TIME ZONE sy-zonolo
INTO DATE startdate TIME starttime.
CONVERT DATE startdate
TIME starttime INTO TIME STAMP L_TSTAMPS
TIME ZONE timezone.
CONVERT TIME STAMP L_TSTAMPS TIME ZONE timezone
INTO DATE L_DATE TIME L_TIME.
.....
endloop.
Srikanth.