Planning help for Python - Time Calculator Project

Hello I need help planning my code for the Time Calculator Project please. The following isn’t any actual code I’m just trying to make sense of it in plain words to make sure I understand it. I’ll be grateful for any input thank you!

The point of the project:

Receives 2 strings
Add duration to start time
Return string

How I think I should start:

Create a seperate function that converts time in general
Use ‘if’ to create range of 60 for minutes/seconds and 1-12 for hours with an increment of 1
12 AM - 11:59AM
12PM - 11:59PM
Should I use try/except? To make sure conversion is only digits?

What I think I should do next:

Def add_time(start, duration):
Convert start
Index and slice at ‘:’
Cast to int
Add numbers to a list or dictionary or tuples and then combine them?
then call the time conversion function while using a for loop?
Add numbers then combine by using join at a ‘:’
Assign added time to new_time variable

How I think I should finish:

New_time = ’ ’ #I need to return a string and this is declared at start of function
Return new_time