Specifically I have code that simplifies to this:

from datetime import datetime
date_string = '2009-11-29 03:17 PM'
format="%Y-%m-%d %H:%M %p"
my_date = datetime.strptime(date_string, format)

# This prints '2009-11-29 03:17 AM'
print my_date.strftime(format)

What gives? Does Python just ignore the period specifier when parsing dates or am I doing something stupid?

The Python time.strftime docs say:

When used with the strptime() function, the %p directive only
affects the output hour field if the %I directive is used to parse
the hour.

Sure enough, changing your %H to %I makes it work.

format="%Y-%m-%d %H:%M %p"

The format is using %H instead of %I. Since %H is the “24-hour” format, it’s likely just discarding the %p information. It works just fine if you change the %H to %I.

You used %H (24 hour format) instead of %I (12 hour format).

Try replacing %H (Hour on a 24-hour clock) with %I (Hour on a 12-hour clock) ?

>>> from datetime import datetime
>>> print(datetime.today().strftime("%H:%M %p"))
15:31 AM

Try replacing %I with %H.