I will give general timings and for specific times you should consult your vaadyaar. Sraaddam should be done during Aparaanna kaalam. This will approximately start after 1pm...needs to be adjusted for sunrise time. So the thithi should be present during this Aparaanna period. If the thithi occurs on two days whichever day has more thithi present during this period should be taken and the other day will be marked as athithi in panchangam. There are a lot of references in Internet about how the day is divided and how this kaalam is calculated. This applies to Amavasyai too since we are doing tharpanam in the place of sraaddam