Incrementing by day fails for timezones like Chile/Continental where spring forward occurs at midnight. Works for Europe.

[quoth-le-corbeau]

import pendulum

# pendulum==3.0.0
# Python 3.10.7

# Step 1 : start the day before spring forward dst day
>>> dt = pendulum.datetime(2025, 9, 6, 0, 0, tz="Chile/Continental")
>>> print("Start:", dt)
Start: 2025-09-06 00:00:00-04:00

>>> print(dt.add(days=1))
2025-09-07 01:00:00-03:00 # since no midnight in chilean tz

# Step 2: Add 1 day and normalize to start_of_day
>>> dt_plus1 = dt.add(days=1).start_of("day")
>>> print("After 1 day:", dt_plus1)
After 1 day: 2025-09-06 23:00:00-04:00

# Step 3: Check if it's truly start of day
>>> print("Is start of day?", dt_plus1 == dt_plus1.start_of("day"))
Is start of day? False

# Step 4: Add another day and normalize again
>>> dt_plus2 = dt_plus1.add(days=1).start_of("day")
>>> print("After 2 days:", dt_plus2)
After 2 days: 2025-09-07 01:00:00-03:00

# BUT Same concept works for CET correctly 

# Step 1: Start one day before spring forward day (but spring forward at 2am not midnight!)
>>> dt = pendulum.datetime(2025, 3, 29, 0, 0, tz="Europe/Vienna")
>>> print("Start:", dt)
Start: 2025-03-29 00:00:00+01:00

>>> print(dt.add(days=1))
2025-03-30 00:00:00+01:00

# Step 2: Add 1 day and normalize to start_of_day
>>> dt_plus1 = dt.add(days=1).start_of("day")
>>> print("After 1 day:", dt_plus1)
After 1 day: 2025-03-30 00:00:00+01:00

# Step 3: Check if it's truly start of day
>>> print("Is start of day?", dt_plus1 == dt_plus1.start_of("day"))
Is start of day? True

# Step 4: Add another day and normalize again
>>> dt_plus2 = dt_plus1.add(days=1).start_of("day")
>>> print("After 2 days:", dt_plus2)
After 2 days: 2025-03-31 00:00:00+02:00

[quoth-le-corbeau]

just to be super clear: the 'bug' for me here is not that we land at 1am chilean time on 07.09.2025. This is actually great and exactly what I would expect from an (otherwise awesome) time-handling library. I added one day from midnight on the 6th and I got the start of the next day, no problem. Great infact, thanks! BUT what sucks is that when I then add one day I do not get to the start of the next day. So pendulum is clever enough to work out that the start of spring-forward day is 1am (midnight in this timezone simply does not exist) yay! but nay! one 'day' afterwards is not just 24 hours later so the cleverness is lost.