If you took your niyam on 19th December, then in normal day-to-day counting:

• One full month (same date to same date)

→ Your niyam completes on 19th January. (You follow the niyam up to and including 18th January; from 19th January you are free, unless your Guru has said otherwise.)

Some people instead do fixed 30 days:

• Day 1 = 19th December
• Day 30 = 17th January

So:

• Calendar “one month” niyam → finishes on 19th January
• Exact “30 days” niyam → finishes on 17th January

In Jain practice, we usually follow how the niyam was spoken (wording) or how Guru Bhagwant / Pravachankar instructed. If you are unsure, the safest is to continue up to 19th January and ask your Guru or elder for a final confirmation.