How to determine shraddha tithi or amavasya tarpanam tithi.

Shraddha is performed when tithi prevails during aprahana kala of the day. The Aprahana kala is the 4/5th part of the day. The Shraddha is performed during the Aprahana kala of the day, after completing the madhyahna sandhya.

1. To calculate aprahana kala subtract sunrise from sunset (dinmana:=sunset-sunrise).
2. The result is called dinmana.
3. Divide it into five equal parts, to do this divide dinamana with 5. This gives us the duration for each kala. Now aprahana kala is the 4th kala of the day. It starts right after 3rd madhyana kala.
4. Multiply kala duration obtained in step #3 with 3 to give us aprahana kala starting offset from sunrise.
5. Add sunrise to aparahna kala starting offset. This is the begining time for aprahana kala.
6. Add the duration of kala (Step #3) to the aprahana kala start time calculated above (Step #5). This is when aprahana kala ends.
7. Now check the tithi if it prevails during the aprahana kala that is from the start of aparahana kala obtained from step 5 to time obtained in step 6. To find tithi ending times and local sunrise and sunset please visit http://www.mypanchang.com/ and click on panchanga, scroll down to your city and click on panchanga. There are two columns, one for Amavasyanta panchanga and another for poornimant. Please click panchangam under appropriate calendar you follow.
8. If your tithi prevails during this period, It is your tithi. Repeat the same for the next day to check if tithi prevails during next day, If your tithi or amavasya prevails during both days then take one in which prevails more.

Now this entire calculation should be done on local sunrise and local sunset -- Where the shraddha or tarpanam is going to be performed. If you are in america and the priest from India gives you the date based on their sunrise sunset and their tithi timings it's invalid. You might be performing it on a wrong day. To find tithi end times and local sunrise and local sunset in your city -- location please visit http://www.mypanchang.com/ and click on panchanga, scroll down to your city and click on panchanga. There are two columns, one for Amavasyanta panchanga and another for poornimant. Please click panchangam under appropriate calendar you follow. If you are from gujarat, maharastra or south india please use amavasyant panchangam otherwise if you are from north india select poornimanth panchangam.