Don’t worry about being precise to the second! DeskTime uses a 15-minute buffer (grace) period for the arrival and leave times.
If you're 15 minutes early or stay 15 minutes late, it all counts toward your shift. Just keep in mind that any time beyond that 15-minute mark will not show up in your shift reports.
Starting later or leaving earlier than the appointed shift time will show arrived/left time in red in the My Desktime section.
Data about employees being late or leaving early can be found in the Reports section.