Change formula if cell not numerical

Good afternoon all

Merry Christmas and Happy New Year to you all

Looking for some help with the ISNUMBER Function,, I have a range of cells with this formula,, it works very well giving me the result I require,, this is to return the hh:mm minus 15 minutes

=IF(A8="","",IF(D8="-","",DGET($D$841:$E$1169,2,D7:D8)-0.25))

What I'm looking for is if the cell D8 is not numerical then do not take the 15 minutes off the time.. ie run this formula...

=IF(A8="","",IF(D8="-","",DGET($D$841:$E$1169,2,D7:D8)))

I have been trying to make it work using the ISNUMBER function but cant get it to work,,,

Many thanks

JT