calculating in weeks

Thank you for clarifying. This code (a model, not a solution) should give you the ideas you need to get the answer you  want:

// Let's suppose the calculation is for 23 days, in total days

total_days=23
// Calculate exact_weeks, the exact number of weeks (including decimal places)
exact_weeks = total_days / 7 ;

// Round this down to a whole number of weeks in whole_weeks
whole_weeks = Math.floor(exact_weeks) ;

// To calculate the days over, first work out how many days in these whole weeks (whole_weeks_as_days)
whole_weeks_as_days = whole_weeks * 7 ;

// Now work out days_over, the extra days in the current week
days_over = total_days - whole_weeks_as_days ;

Look at:

https://community.adobe.com/t5/acrobat/calculating-age-in-weeks-and-days/m-p/10765166

Coders can't work with "one week or two". Code cannot be written until the rules are precisely known, even if that is stricter than people need.  (Also, such a calculation has very serious legal implications, and should not be left up to coders to pick something at random!!) 

You have to decide exactly how you want to calculate "difference in weeks", there are different ways. How would you do it on paper? For example, Monday 1 to Monday 8. One week or two? Friday 5 to Thursday 11. One week or two? 10 am on Monday 1 to 9 am on Monday 8. One week or two?