How to write a formula for a calculated field that subtracts a number from a time field in the table

[Course]

In a calculated field I want to subtract a decimal number which occurs in a lookup field(called UTC Lookup) from the result of this formula [Arrival Time]-[Departure Time]

I tried this [Arrival Time]-[Departure Time]-[UTC Lookup]/24

It returns an error message saying #Type

Can anyone help with this.

Thanks

 

[ranman256]

time math is done with
DATEDIFF("h", date1, date1)