Julian Day Calculation
The formula in the wikipedia article explicitly uses integer division with round toward zero. Python's integer division uses round toward negative infinity (i.e., floor).
The wikipedia article formula repeatedly uses (m-14)/12. This evaluates to -1 for months 1 and 2 (January and February), zero otherwise. You can use the wikipedia article formula in python3 if you change every instance of (m-14)/12 in the wikipedia article formula to any of the following:

int((m-14)/12)The single / performs floating point division. Python's built-in int function rounds toward zero.
((m+9)//12-1)The inner (m+9)//12) evaluates to 0 for January and February (m=1 and 2), 1 for all later months.
(-1 if m&lt;3 else 0)While more verbose, this does a better job of conveying the intent of the wikipedia article's (m-14)/12.