help regarding storing time difference values in same unit?

Dear R-Experts, 

Seek your help.

In a List object e.g. Time_duration[[]], I am storing differece between two
time-stamps (start-time and end-time) for a certain event. 
Time difference does not remain constant for, say, 10 observations.
Each time, the time difference is given in different time units: seconds /
hours / minutes.
Can you please tell me - how I can store the values of time difference in
just one time unit ?
It can be either seconds, or minutes or hours. The criterion is - unit
should be consistent.

Thank you in advance.

Regards,
Suresh

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