contemporaneous charitable receipt needed by 4-15 ?

[RWG1950]

It is my understanding that when a taxpayer has made a large church or charitable contribution but the receipt doesn't have the "magic language", they have to get this corrected on a new receipt and have it in hand by the time they file their tax return.
If they aren't able to get this new corrected receipt by 4-15, will filing an extension until they get this new receipt be the thing to do ? Also, if a corrected receipt is provided but is dated 4-17 and the taxpayer files on 4-18 (under extension) will this satisfy the contemporaneous rule or must the receipt be dated 4-15 or earlier ?
Were close to the filing deadline & my concern is that some of the corrected receipts we've requested from the charities may not be in the taxpayer's hand by 4-15.

[KBTS]

From page 4-20 of TTB:

Required written acknowledgements from the charity must be received by the taxpayer by the earlier of the date of filing or due date of the return, including extensions.

So they need the receipt in hand prior to filing, but you could file an extension if you are waiting for a corrected receipt.

[RWG1950]

Thanks KBTS.