[Python] How to get a billing cycle period between the 26th of the previous month and the 25th...

The Problem


I'm building a billing system in Django, and I need to calculate the billing period for each invoice.

Our business rule is simple:

• The billing cycle starts on the 26th of the previous month at midnight (00:00:00);
• And ends on the 25th of the current month at 23:59:59.

For example, if the current date is 2025-07-23, the result should be:

start = datetime(2025, 6, 26, 0, 0, 0)
end = datetime(2025, 7, 25, 23, 59, 59)


We're using Django, so the dates must be timezone-aware (UTC preferred), as Django stores all datetime fields in UTC.

The problem is: when I run my current code (below), the values saved in the database are shifted, like 2025-06-26T03:00:00Z instead of 2025-06-26T00:00:00Z.

What We Tried


We tried the following function:

from datetime import datetime, timedelta
from dateutil.relativedelta import relativedelta

def get_invoice_period(reference_date: datetime = None) -> tuple[datetime, datetime]:
if reference_date is None:
reference_date = datetime.now()

end = (reference_date - timedelta(days=1)).replace(hour=23, minute=59, second=59, microsecond=0)
start = (reference_date - relativedelta(months=1)).replace(day=26, hour=0, minute=0, second=0, microsecond=0)

return start, end


But this causes timezone problems, and datetime.now() is not timezone-aware in Django. So when we save these values to the database, Django converts them to UTC, shifting the time (e.g., +3 hours).

We looked at:

• this answer from Mitghi
• and this question, which was closed

But none of them solves the business logic and works well with Django timezone-aware datetimes.

What We Expected


We expect a function that:

• Returns a tuple with two datetime objects;
• The first is the 26th of the previous month, at midnight;
• The second is the 25th of the current month, at 23:59:59;
• Both datetimes are timezone-aware (pytz.UTC or Django's timezone.now() preferred).

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