Time and dates are numbers with special formatting. Since they are stored as
numbers you can treat them as such. Once you understand that, most of your
problems will go away. Think of time as portions of a day. 6:00 AM is
stored as 0.25. 6 hours is 1/4 of a day. Just remember to input time as
time (ie 30min should be entered as 00:30). If you a time output in decimal
hours (6.5 instead of 6:30) you need to multiply that cell by 24 and format
it as a number.

Example 1:
Input
A1: 0:30
A2: 0:30
A3: =A1+A2
Formatted as (using a custom number format)
A1: \.[m]
A2: \.[m]
A3: [h]
Will look like this
A1: .30
A2: .30
A3: 1

Example 2:
Input:
B1: 00:40:00
B2: 6:30:00
B3: B1+B2
Custom Number Format
B1: [m]
B2: h:mm
B3: h:mm
Will look like
B1: 40
B2: 6:30
B3: 7:10

Example 3:
Same as Example 2 except
C3: =C2-C1
Will look like
C1: 40
C2: 6:30
C3: 5:50