This feature is accessed in the Global Settings, from Settings > General > Global and applies to all time clocked, the start and end time.
This rounds that clock time, to the nearest interval bracket selected by you.
For example, If you selected 15 minute rounding and Stephen was meant to start at 9:00
If Stephen clocked in at 09.05 it would be rounded down to 09:00am as that is the nearest 15 minutes interval, and he would be on time.
However if Stephen instead clocked at 09.12 his time would instead be rounded up to 09.15 as that is now the nearest interval.