Intergers and Remainders

If you are trying to do this all in one cell, you can try the duration format like you were thinking. Format as shown below and enter your value as 17d. The math to go to the next day would be +"1d" .

If you have the decimal number 17 representing the number of days and you want to express that in weeks and days then you have to convert from Number to Duration first, using the DURATION function. Then you can apply the custom Duration formatting you want.

=DURATION(0,A2)

You need to leave that leading 0 or the weeks placeholder in there.

Or if you want a more verbose presentation (or need to export to Excel) then you can use the first method I suggested above, without first converting to Duration, something like this:

Substitute ; for , in the formulas if your regions uses , as a decimal separator.

SG

If whatever is in the cell already can't be turned into the format you are trying to select, it won't hold. If I type 17 into a cell then change the format to duration, it turns it into 2 weeks 3 days but if I have the formula =B2+1 in the cell, where B2 is a number, it is a numeric formula that will not convert to a duration.

So, if you have formulas in those cells, you may have to rework the formulas into duration formulas or do as SGIII says and use the DURATION function to convert them.

Depending on what you are doing you may find it easier to work with the decimal numbers and convert to Duration at the end.

But if you want to rework your calculations to express everything in Duration and you're have trouble getting a 17 (or whatever) that you've entered to convert to Duration (the formatting doesn't "stick") then keep in mind that you can unambiguously tell Numbers you mean Duration by using special input notation. If you haven't pre-formatted the cell (as say, Text) then just enter 17d and Numbers will treat that immediately as Duration. w, h, m, s, and ms are also valid notation for entering Duration.

SG

Are you trying to do something like this?

If so, the QUOTIENT and MOD functions are your friends. More here and here.

SG