In Jain practice, a “1 month” niyam is normally understood as 30 days, not strictly “date to date.”

So in your example:

• You take niyam on 20th December for “1 month”
• You then count 30 full days from 20th December
• Your niyam will usually complete on the morning of the 31st day (i.e. after 30 days are over)

Because:

• Many traditional Jain tapas/vrats (like Masakshaman – month‑long fast) are defined as 30 days, even if the calendar month has 31 days.
• The main thing is: you clearly say in pachchakkhan (saṅkalp) what you mean by “1 month.”

To be safest and in full maryādā:

1. While taking niyam, say clearly:

- “I take this niyam for 30 days” or - “I take this niyam till 20th January morning.”

1. Then follow exactly what you spoke in pachchakkhan.

Local traditions (and your guru / pravartini / pandit) may have a set rule in your sangh (some may say date‑to‑date, some 30 days). If they have a fixed practice, follow that first.