ios numbers convert cell with number of days “54d” to plain number “54” so I can multiply against it

I’m trying to calculate how many gallons of water I run through a filter.

i know the number of days, and I know how much water I am putting in to be filtered

(Yes I can do this in calculator but I want to be able to see how many gallons total if I adjust the days and/or the amount of water added.)

See screenshot with red circles cell.

I subtract b2 from b3 to get total days in b4. i can’t find a cell formatting option to convert “54d” to just “54” so I can multiply in a simple formula to get my total gallons run through a filter in e5 that doesn’t show as duration.

I can manually enter the total days in d5 (see second screenshot with black circled cell) but it’s a spreadsheet not a Lego… any help appreciated. I’m trying to dump excel for home and business.