Convert a CRON expression from local TZ to UTC in Python

Question

I'm building an application in which some tasks are scheduled by using CRON like jobs (with celery). Through an interface, jobs are saved as celery jobs with django-celery-beat package.

For that, the schedule the users want is translated into a CRON expression such as 30 9 * * 1 (Every Monday at 9:30). It works well. But the celery backend as well as the database are working in UTC timezone. While end users of my application way work from a different timezone.

So if a user is working in UTC+1, and he wants the job to be run at 9:30 each Monday, it's not 30 9 * * 1 that will have to be used but 30 8 * * 1 instead.

This use case is kinda easy to solve but what if I want it to be run at 00:00 on each Monday ? It means that I'll have to go a day backwards, so Sunday at 23:00 which would lead to 0 23 * * 0. And the same behavior for every possible timezone other than UTC.

Is there an easy way to do so ?

Also I'm worryied about DST...

Thanks in advance !

Answer 1

There's two things here.

• first is changing the hour value, which can easily be achieved via:

    # Hour difference
    h_diff = 1
    # Original Crontab
    cron = "30 8 * * 1"
    
    # Fixing hour
    cron_list = cron.split(" ")
    h_val = cron_list[1]
    corrected_hour = (h_val - h_diff) % 24
    cron_list[1] = corrected_hour
    
    # Recreating Crontab
    new_crontab = " ".join(cron_list)

• As for Daylight saving time, as said in this post: Daylight savings time in Python

You should convert to UTC and stay there unless printing for users. However you might have to redefine your crons daily to be able to catch DST changes.