An overview of SYST constants is available to you any time. With a small demo report, which highlights / explains timezones.

There are quite a few system constants available concerning dates and timezones:

  • SY-DAYST Daylight savings time, indicates "summertime" (+1 hour) or not
  • SY-TZONE Time difference to the UTC reference time in seconds, ignoring daylight savings time.
  • SY-TIMLOTime in the current user's time zone.
  • SY-ZONLO User's time zone.
  • SY-UZEIT The actual time, in which the SY-TZONE will have been processed. Use statement GET TIME in your Abap logic to refresh this time
  • SY-DATUM Today's date.
  • SY-DATLO Date in the current user's time zone.
  • SY-FDAYW Factory calendar weekday. "1" for Monday, ..., "5" for Friday.

The example  report below will simply output the system fields to a report. 

write:
/ SY-DAYST, 14 'Daylight savings time, indicates "summertime" (+1 hour) or not',
/ SY-TZONE, 14 'Time difference to the UTC reference time in seconds, ignoring daylight savings time.',
/ SY-TIMLO, 14 'Time in the current user''s time zone.',
/ SY-ZONLO, 14 'User''s time zone.',
/ SY-UZEIT, 14 'The actual time, in which the SY-TZONE will have been processed.',
/ SY-DATUM, 14 'Today''s date.',
/ SY-DATLO, 14 'Date in the current user''s time zone.',
/ SY-FDAYW, 14 'Factory calendar weekday. "1" for Monday, ..., "5" for Friday. '.

Which would result in

             Daylight savings time, indicates "summertime" (+1 hour) or not
3.600        Time difference to the UTC reference time in seconds, ignoring daylight savings time.
14:21:28     Time in the current user's time zone.
CET          User's time zone.
14:21:28     The actual time, in which the SY-TZONE will have been processed.
14.11.2016   Today's date.
14.11.2016   Date in the current user's time zone.
 1           Factory calendar weekday. "1" for Monday, ..., "5" for Friday.

Note the 3.600 on the SY-TCODE field, it's the number of seconds between system time in system timezone (CET in this example) and the user time. This could for example play a role when your user changes something on a PO: the change date/time will reflect the user date/time. However if something is printed, the SPOOL creation time will reflect an hour earlier, the system time.