Computation of years between periods

let’s say i have a buy and a sell.

Buy@$100 in 2010-06-03

Sold@167 in 2015-09-04

In order to calculate CAGR i need the period right? I’ve been using the period as years using a 365 days year but i’ve seen it made in several other ways. Years of 360 days, 365.25 days, number of complete years + a fractional year.

Is there a GIPS standard? I can’t seem to find any. It seems such a simple question and yet i can’t find anything definitive about the calculation.

Thank You