You should really provide more information. I.e. how are you entering these
times? Are they in cells as times (i.e. with colons)? I'll assume so.

Therefore your elapsed time  calculation will be something like

=F5-F4

Seeing there is no date information in the date-time fields, you need to
compensate. Basically, you need the result modulo one day. I.e.

=MOD(F5-F4,1)

If you're holding the times in some other fashion, you'll need to go modulo
24 hours or one day, as appropriate.