Simple math

I have just got the hour using function 2c of int 21h, and the hour is in ch. Now what i want to do is find out if the hour is above 12, and if it is, subtract twelve from it, any ideas on how to do that?

[itay222]

what am i missing here?

cmp ch, 12
jb dontsub (or jl... it has been a long time)
sub ch, 12
dontsub:

[Cahlel]

I'm pretty sure that they both work, JB and JL that is