Help needed: Calculated field Question

Hi All

I have a calculated field that counts hours between 2 date/time fields
here is my formula
=TEXT([date of cover]-coverdate_till,"hh:mm")

but it is calculating when there is no date entered in the 2nd Date/Time field and i need it to show zero

Any help greatly appreciated
Gary
Ps i know nothing about calculated field formula other than a basic level