Determining the Amount of days in each month in a week that crosses Months

[MDuff]

For example I have data that's stored by weeks but I would like to know what % of that data belongs in a month

So Using a Monday Start for the week of 9/28/2015 I would have 3 days in month 9 and 4 in Month 10

Does n any one have a formula I can run on 9/28/2015 to tell me that 3 days are in September and 4 days are in October?

thanks

 

[Dendro]

Let's say you would have the date 9/28/2015 in cell A1:
1. put this formula in B1, which will show the remaining days of that week in September:

=[COLOR=#ff0000]DAY(DATE(YEAR(A1);MONTH(A1)+1;1)-1)[/COLOR][COLOR=#006400]-DAY(A1)[/COLOR][COLOR=#0000ff]+1[/COLOR]

-this part gives the amount of days in the month of the date in A1 (30), by taking the first day of the next month (march), subtracting one day, and taking the day number of this date(9/30/2015 -> 30)
-subtract the day number of A1 (28)
-we should count Monday as well, therefore +1
2.put this formula in C1, which will show the remaining days of that week in October:

=7-(DAY(DATE(YEAR(A1);MONTH(A1)+1;1)-1)-DAY(A1)+1)

-subtract the amount of days in the week of September from a week of 7 days