Time zone issues between Python and Django

When time issues are involved in coding, it is always easy to be confused by the time zone issue. I have always dealt with it through trial and error. Today I finally figured out the reason and resolved a knot in my mind.

Python’s time zone problem

datetime.today() / datetime.now()

These two functions obtain the current system time, but the tzinfo in the obtained datetime object is empty, even if it is set in the system Time zone.

datetime.utcnow()

This function gets the current UTC time, which should be calculated based on the current system time and time zone.

For example, if the system time is 14:00, the time zone is Asia/Shanghai (Beijing time), the utcnow return time is 6:00. Likewise, the tzinfo in the resulting object is empty.

The impact of environment variable TZ on the above functions:

When the environment variable TZ is set in the system, or when os.environ['TZ'] is set in python, the time obtained by the above function is TZ The time corresponding to the time zone. In fact, it can be considered that TZ affects not these functions, but the system time, which can be seen from the return result of the date command. The results returned by datetime.now() and the date command are always consistent.

Django’s time zone problem

After understanding the above functions in python, Django’s time zone problem seems simple.

In the django setting, there is a setting called TIME_ZONE to set the time zone used in the program.

We know from the django documentation that the function of TIME_ZONE is to change os.environ['TZ'], but changing os.environ['TZ'] will not change the system environment variable TZ. Therefore, if TIME_ZONE is set to If the system time zone setting is inconsistent, the time obtained by datetime.now() in the program will be inconsistent with the time of the date command.

Therefore, TIME_ZONE should be set to the time zone the program wishes to use. For a local program, TIME_ZONE can be set to the same as the system time zone; for an international application, it is best to set TIME_ZONE to UTC, and then adjust it according to the current user's time zone when displaying.

Manual

classmethod datetime.now([tz])

Return the current local date and time. If optional argument tz is None or not specified, this is like today(), but, if possible, supplies more precision than can be gotten from going through a time.time() timestamp (for example, this may be possible on platforms supplying the C gettimeofday() function).

Else tz must be an instance of a class tzinfo subclass, and the current date and time are converted to tz's time zone. In this case the result is equivalent to tz.fromutc(datetime.utcnow().replace(tzinfo=tz)). See also today(), utcnow().

classmethod datetime.utcnow()

Return the current UTC date and time, with tzinfo None. This is like now(), but returns the current UTC date and time, as a naivedatetime object. See also now().

Time zone conversion Code

import pytz
....
#dt the datetime var
dt.replace(tzinfo=pytz.utc).astimezone(pytz.timezone('Asia/Shanghai'))