It's the date for T2.
You can set the date separately for both time zones.
At least it's so in both my delta and kappa.
T2 date setting is a bit tricky:
If you enter setting mode from normal display mode (time+date shown), you can only set the date and time for T1, even if T2 is on display.
To set the date for T2:
1. scroll to T1+T2 display mode,
2. press scroller to bring T2 to the first row (that is, "T1" is shown in the second row),
3. press scroller ~3s to enter setting mode,
4. et voilĂ , now you can set both date and time for T2.
If you enter setting mode while "T2" is in the bottom row, you can only set the time for T2, but not the date (this is called "quick set" in the manual).
It does make sense to have a separate date for T2 to make the day change over at midnight according to T2, not T1.
But I don't know why there are 2 different T2 setting modes...
BTW I use T1 for normal time and T2 for DST. So DST switchover is as simple as "scroll one step reverse, press scroller, scroll one step forward - done"