On time, late, early if statement

Hi,

I am trying to create an IF statment to show LATE, EARLY, and ON TIME but I want a grace period of 30mins

CURRENTLY I have.

=IF(Y18>X18,"Late",IF(Y18<X18,"Early","On Time"))

DATA:

PLANNED ARR TIME	ACTUAL ARR TIME	LATE OR ON TIME
1700	1637	Early
1600	1554	Early

Any helps would be great.