Since you quote adhikar maasam(or adhika masam) you are following hindu calendar and not tamil Calendar and hence as per that the date you specified will be equivalent month of thai in tamil (pausa/pushya/magara in different regions) and in 2018 to 2019 also there was adhika masam and hence it should not be taken as dhanur that is marghali in tamil. So that year pushya/magara started in 5th January 2019 and your father passed away on 7th January 2019.Namaskaram Sri Gopalan avarhale!
There is a small confusion in the family & some are saying that since there is adhikaar maasam this year, my father's tithi should come in January next year & not in December as calculated by you.
Can you please clarify this doubt?
Thanks and regards.
Though thithi comes on 15th Jan srartha thithi on 14th Jan itself.thithi comes in 15th January 2021