dima

The problem

You have a periodic task in Celery defined with a crontab(...) schedule, and you want to calculate the next time it's supposed to run.

Example: you want to find out when crontab(hour=12, minute=0) will trigger next after now.

Simple, right? There’s croniter library, which seems to be designed to solve this exact problem. Just use it, right?

Well.

First mistake: trying croniter with crontab

So my first instinct was to use croniter like this:

from celery.schedules import crontab
from croniter import croniter
from datetime import datetime

schedule = crontab(hour=12, minute=0)
cron = croniter(schedule, datetime.now())
next_run = cron.get_next(datetime)

Boom 💥 doesn’t work. Because Celery’s crontab is not a string and croniter expects a string like "0 12 * * *":

AttributeError: 'crontab' object has no attribute 'lower'

And no, crontab() does not have a nice .as_cron_string() method either.

So now you’re stuck parsing crontab's internal fields (._orig_minute, ._orig_hour, etc) just to reconstruct a string - and it starts to smell like overengineering for something that should be simple.

The right way (which I learned too late)

Turns out Celery’s crontab (and all schedules derived from celery.schedules.BaseSchedule) already has a method for this:

from datetime import datetime
from celery.schedules import crontab

schedule = crontab(hour=12, minute=0)
now = datetime.now()
# `now` is datetime.datetime(2025, 6, 11, 0, 16, 58, 484085)
next_run = now + schedule.remaining_delta(now)[1]
# `next_run` is datetime.datetime(2025, 6, 11, 12, 0)

That’s it. You don’t need croniter at all. Celery knows how to calculate the delta to the next run. It just doesn’t shout about it in the docs.

Summary

• don’t reinvent Celery’s scheduling logic - it already has what you need;
• crontab is not a cron string, don’t try to treat it like one;
• use .remaining_delta(last_run) to calculate when the next run will happen.

Hope this saves someone the 2 hours I wasted trying to do it the wrong way.