Comparing times
In a nutshell
You can use seconds, minutes and hours to not only give times of events but also to calculate the duration of events. This enables you to calculate their length and you can compare how long different events last, to fit into a daily routine.
Hours, minutes and seconds
Seconds are the smallest unit of time, then minutes and then hours. Use the following equations to convert between them:
$1 \text{ minute} = 60 \text{ seconds}$
$1 \text{ hour} = 60 \text{ minutes}$
Example 1
How many seconds are in $1\text{ hour and } 4\text{ minutes}$?
$1 \text{ hour} = 60 \text{ minutes}$
$60 + 4 = 64 \text{ minutes}$
$64 \times 60 = \underline{3840 \text{ seconds}}$
Comparing times
To obtain the duration of an event, you need to compare the start and ending time of the event.
Procedure
1.
 Count how many full hours are between the start time and ending time. 
2.  Count how many full minutes are between the last full hour and the ending time. 
3.  Count how many seconds are between the last full minute and the ending time.

Example 2
A concert starts at $18:30:20$ and ends at $21:10:45$. What is its duration?
Number of full hours between $\underline{18}:30:20$ and $\underline{21}: 10: 45$ is $2$.
Number of full minutes between $18:\underline{30}:20$ and $21:\underline{10}:45$ is $40$.
Number of seconds between $18:30:\underline{20}$ and $21:10:\underline{45}$ is $25$.
Hence the concert has a duration of $\underline{2\text{ hours and } 40 \text{ minutes and } 25 \text{ seconds}}$.
Note: There are only two full hours between $18:30:20$ and $21:10:45$ as you can see that the number of minutes in the second time is smaller than the first.