It's 10 after/past 12.

[wotcha]

So my grammar book says we can't say

'It's 10 past 12'.


but

'It's 10 after 12'? is possible.


Is that correct?

[5jj]

"It's ten past twelve" is natural and correct in BrE.

[bhaisahab]

If your grammar book says that "It's ten past twelve" is wrong, it's a very poor grammar book.

[SoothingDave]

"It's ten past twelve" is natural and correct in BrE.

And in AmE. "Ten after twelve" is also OK. Or "twelve ten."