In Ireland at least, and maybe in England also (I don't know), there was a fine for registering the birth late, so people seemed to give a later date than the actual one when they went to register the birth civilly.
And - he had to be born before he was baptized. - That is one thing I go by.
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edited to correct grammar and to add:
How close together are the dates on the baptism record? It's hard to get it too far wrong if he was born one day and baptized the next.
When I put the information into a genealogy application, I usually pick the most likely date, then add a note mentioning the others. You (we)'re really trying to guess how people were thinking at the time, which is the best we can do.
And as GR2 mentioned, you might get still another date from the 1939 register and any other later records.