How to get milliseconds after midnight with the hours, minutes and seconds we give in the function? (in Python)

Like this

import datetime

def past(h,m,s):
    return datetime.timedelta(hours=h, minutes=m, seconds=s).total_seconds()*1000

This will convert the timedelta object into seconds and then multiply it by a 1000 to get the milliseconds.

Alternatively you could just use maths

def past(h,m,s):
    returns (h*(60**2)+m*60+s)*1000

Which is equal to (hours*seconds_in_hour + minutes*seconds_in_minutes + seconds) * millis_in_second