Retrieve last possible execution datetime of a celery task

crontab = settings.CELERYBEAT_SCHEDULE['mytask']['schedule']
# We get the duration until the next execution
delta = crontab.remaining_estimate(crontab.now())
# We get the datetime of the next execution
next_exec = datetime.now() + delta
# We get the duration between two execution
duration = crontab.remaining_estimate(next_exec) - delta
# So the last run is the next_exec minus the duration between two task.
last_run = next_exec - duration