A new task for you!
You have to create a method, that corrects a given time string.
There was a problem in addition, so many of the time strings are broken.
Time-Format is european. So from "00:00:00" to "23:59:59".
Some examples:
"09:10:01" -> "09:10:01"
"11:70:10" -> "12:10:10"
"19:99:99" -> "20:40:39"
"24:01:01" -> "00:01:01"
If the input-string is null or empty return exactly this value! (empty string for C++)
If the time-string-format is invalid, return null. (empty string for C++)
Have fun coding it and please don't forget to vote and rank this kata! :-) 
I have created other katas. Have a look if you like coding and challenges.
import re

def time_correct(s):
    try:
        assert re.match(r"^\d\d:\d\d:\d\d$", s)
        a, b, c = [int(x) for x in s.split(":")]
    except:
        return "" if s == "" else None
    b += c // 60
    a += b // 60
    return ":".join("{:>02}".format(x % y) for x, y in zip([a, b, c], [24, 60, 60]))
import re

def time_correct(s):
    try:
        assert re.match(r"^\d\d:\d\d:\d\d$", s)
        a, b, c = [int(x) for x in s.split(":")]
    except:
        return "" if s == "" else None
    b += c // 60
    a += b // 60
    return ":".join("{:>02}".format(x % y) for x, y in zip([a, b, c], [24, 60, 60]))