Greater than in if statements?

It seems to want to reference a cell when I use ">"

I basically want it to say
=IF(calculate!K21=0, " ", if( calculate!K21>12:59:59, calculate!K21-12:00:00, calculate!K21))
but it says there's an error

That way I can have the cells always return 12 hour increments instead of military time and return nothing but a space if there's a zero value in the referenced cell.