DateDiff not working when finding amount of years between two dates - vba

Question

I've got the following formula
number_of_years = DateDiff("yyyy", Date_days_back, Current_Date)
where
Date_days_back = 17/10/2020
Current_Date = 02/08/2021
for some reason this returns "1" when there aren't 365 days apart between these two dates, is something wrong with my formula or is DateDiff calculating this wrong? I would like to get "0" returned as there aren't enough amount of days between these two dates for a year to have passed between the two dates.
Comment: As Scott Stated, if I'm looking from December 31st 2020 and 1st of January 2021, then this function will count this as a year has passed. is there another function that calculates if a years has passed in between two dates where it doesn't do what DateDiff does and instead counts if 365 have passed in between the two dates?

Answer 1

Try to use YearFrac(). By setting the third argument to 1, the function uses the actual number of days in years (taking into account leap years) in its calculations:
Int(WorksheetFunction.YearFrac("2020-12-31", "2021-12-31", 1))
Comparison:
Sub Example()
start_date = CDate("2020-12-31")
end_date = CDate("2021-01-01")
Debug.Print Int(WorksheetFunction.YearFrac(start_date, end_date, 1))
' prints 0
Debug.Print DateDiff("yyyy", start_date, end_date)
' prints 1
End Sub

Answer 2

This seems to work
function yeardiff(d1 as date, d2 as date) as integer
yeardiff = datediff("yyyy", d1, dateadd("yyyy", -1, dateadd("d", 1, d2)))
end function
Tests
print yeardiff("2020-12-31", "2021-12-30")
0
print yeardiff("2020-12-31", "2021-12-31")
1
Of course it will also give,
print yeardiff("2020-12-31", "2020-12-30")
-1
And if something different is desired then a conditional would be needed.