ZoneInfo does not process AT field suffix. and this causes cutover time is not correct in many countries.
I can see the authur didn't understand what those suffix are from code comments.
Here is a descripption from zic.8.txt:
AT Gives the time of day at which the rule takes effect.
Recognized forms include:
2 time in hours
2:00 time in hours and minutes
15:00 24-hour format time (for times after noon)
1:28:14 time in hours, minutes, and seconds
- equivalent to 0
where hour 0 is midnight at the start of the day, and hour 24
is midnight at the end of the day. Any of these forms may be
followed by the letter w if the given time is local "wall
clock" time, s if the given time is local "standard" time, or u
(or g or z) if the given time is universal time; in the absence
of an indicator, wall clock time is assumed.