InfoPath Dev

Hi, My form has a start and end date picker. I would like to know what would be the formula to calculate the number of days since the end date and number of days between the two. I am new to infopath and only has infopath 2013 and no designer. I tried the following formula [End Date]-[Start Date] now()-[End Date] and it doesn’t calculate anything , just leaves it as NaN. I am searching online and there are some suggestions to use DATEIF but I can’t find this function in info path . Is there a formula where I can input in the formula box that will 1) calculate the number of days between two Dates 2) number for days from End Date til now? Many thanks!!

There isn't anything like this out of the box, which is awful, I know. There are blog posts that describe how to do this with some helper fields - here's one: http://www.bizsupportonline.net/infopath2007/calculate-date-difference-infopath-rules-formulas.htm 

For number of days between 2 dates, I'd use calculated field column type and put the DATEDIF formula in the column setting in the SP list setting..not in info path designer. Then for number of days between today and a date, I'd create a date column in my list, then have a site workflow to update that date column everyday. There's other option of using job/task scheduler, but I don't know how.