How to accurately/precisely count number of months, days from a particular date to a date

Respected All,

I tried different methods, told by colleagues or shared by experts on different forums. But I was looking for a precise/accurate method to calculate number of months, days from a particular date to an ending date e.g. 1-Oct-2014 to 1-Sep-2015. I will use this information to make payments. Any support in this matter will be highly appreciated please.

Regards