When must one perform tharpanam during sankramana punya kalam? Must this be done during afternoon (aparAhNa) like amavasai? Or must this be done when Sun enters a particular rAsi? For example: In this year’s Srirangam Vakhya panchangam, Dakshinayana punya kalam is said to occur at kata ravi 45-13 nazhiagais. If so, this means the time is 17 July 12 AM.

Sankramana punya kala tharpanam is performed when the month comes into being. If a punya kalam begins morning 7 AM, after Sunrise and performance of sandhyavandanam, tharpanam must be performed.
Notes:
If we consider Adi masam, scriptures say that Dakshinayana punya kala tharpanam must be performed in Uttarayanam. So, even if the month begins night/midnight on that concerned day, if time is available before Sunset, tharpanam can be performed, based on the pramanam “yAyAnatsannihitahA”. This means, those who can, can perform the tharpanam as much closer to the time of the punya kalam. Those who are not able to can perform the tharpanam during aparAhNa kalam.