In the Excel date system, one day is equal to 1, so you can think of time as fractional values of 1, as shown in the table below:
Since there are 24 hours in a day, 60 minutes in each hour, and 60 seconds in each minute, you need to divide by 24 * 60 * 60 = 86400 in order to convert decimal seconds to a value that Excel will recognize as time. After dividing by 86400, you can apply a time format of your choice, or use the result in a math operation with other dates or times.
In the example, since B11 contains 43200 (representing 43200 seconds, or a half day) the result is 43200/86400 = 0.5. Once a time format like h:mm or [h]:mm is applied, Excel will display 12:00.
Displaying a time duration
To display hours that represent a duration longer than 24 hours, minutes in durations longer than 60 minutes, or seconds in durations over 60 seconds, you'll need to adjust the number format by adding square brackets.
[h] // for hours greater than 24 [m] // for minutes greater than 60 [s] // for seconds greater than 60
The brackets tell to Excel the time is a duration, and not a time of day.
Note: to use square brackets, you'll need to create and apply a custom number format. Select cells, then go to Format Cells (Control + 1) > Number > Custom.