Excluding text values

I am making a schedule sheet and have this formula in place to add up time for week.

=SUM((C8-B8)*24)+((E8-D8)*24)+((G8-F8)*24)+((I8-H8)*24)+((K8-J8)*24)+((M8-L8)*24)+((O8-N8)*24)

I am looking for a way to exclude text, or set text equal to zero if i type Text into the the cell where i have my time, "Event, OFF, etc.." Now when i add text to one of those blocks i get a #value! error.

Any help would be greatly appreciated Thanks in advance!!!