in a part of my application the user needs to select two dates in the datepicker
and I need to save the number of days between these two dates
but the months do not have equal days how do I return the interval between different months?
That is done automatically if you use the right blocks for it.
Use the DURATION TO DAYS block. Attach a DURATION (start,end) to it with the start and end dates.
Also, your procedure assumes that both months are in the same year. Just use the built in blocks and enjoy a nice day! 
thaaannnkss
but I only had one problem: the duration does not accept dates apparently what do I do?
convert the dates into an instant of time, see the documentation of the Clock component
Taifun
sure !
thanks 
Well, if it doesn’t accept dates, then we are in trouble because that’s the only purpose of it! 
Just kidding!
Learn all about instants, durations, milliseconds, etc. It sounds complicated but it will save you a lot of time when working with dates in Kodular/App Inventor.
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or give a try to extension by @vknow360
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