How I got to work:
DATETIME_DIFF({Today’s date}, {Date of the first invoice}, ‘days’)/365
I tried the following, hoping to get decimal, such as 4.5 but result was just 4 or 4.0 or 4.00 (depending on Formatting.):
DATETIME_DIFF({Today’s date}, {Date of the first invoice}, ‘years’)
So, I had to use days and then divide by 365 to get 4.5, etc.