For calculating the difference between two times, the time values should be converted to an epoch value first. This  can be done using the Asterisk function STRFTIME. Use the below dialplan to convert current time to an epoch value

Set(time=${STRFTIME(${EPOCH},,%s)})

Below dialplan shows, asterisk setting two variables starttime and endtime with the current time in an interval of 10 seconds and then calculating the difference between the variables and saving to variable diff. The output of variable diff will always be 10 in the below dialplan

exten => 1000,1,Set(starttime=${STRFTIME(${EPOCH},,%s)})

same => n,Wait(10)

same => n,Set(endtime=${STRFTIME(${EPOCH},,%s)})

same => n,Set(diff=$[${endtime} -${starttime}])

same => n,Hangup