hi cartica. you should be doing a MDY conversion. the deciding factor is the format your data is currently in. how you determine is to look at dates bigger than 12. months are never bigger than 12. so you can see some of the data showing numbers bigger than 12 in the middle. hence, it should be MDY. if you have to do a Text to columns every now & then, maybe this would help:
SUMPRODUCT calculates slower though. this is assuming the date is in A2 like your first file & "Buy" in C1. otherwise, you can do a text to column & use Richard's formula:
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