Multiply date with number

[khuslid]

HI folks.
Now I’ve searched for an hour, so I come to you for help:

I have a DURATION -column (nuber of days) that I want to multiply with a NUMBER -column (KWH power consumption) to get the total consumation of power. Both values are variable so I cannot put a fixed value in for calculation.
It seems like APPSHEET won’t accept Date-format to multiply with a Nubmer-format in the FORMULA.

Any workaround suggestions?