Tamil New Year is also known as Varusha pirappu and is observed on first day of Tamil month Chithirai.
In Tamil Nadu when Sankranthi takes place after sunrise and before sunset the year begins on the same day. If Sankranthi takes place after sunset then the year begins on the following day.
This rule is good to start first day of the year for civil reckoning. However auspicious time, for Dan-Punya activities during Mesha Sankranthi, is independent of above rules and should be done after considering exact moment of Sankranthi because only certain time duration before and after Sankranthi moment is considered auspicious for Sankranthi related activities.