[tz] Precise meaning of AT 25:00

[Stephen Colebourne]

The Tokyo rule now includes the value 25:00 in the AT column. Can
someone confirm the exact meaning of the 25:00? Is it

a) an extended local time-of-day
b) the actual duration from midnight at the start of day

While similar, there would be a difference between the two definitions
if there was a second DST gap/overlap earlier in the day.

eg. imagine the case where DST starts at 13:00 (with a gap to 14:00)
and ends at 26:00. With (a) the AT column would hold 26:00, while with
(b) the AT column would hold 25:00

I'm pretty sure it is (a), but wanted to double check before it gets
baked into APIs

Stephen