Alright, this system needs to be robust and easy to use. I shouldn't have to do ANYTHING extra to use this

2 ways to do it
Have a file for each day and clock in and out 
have a master log and parse the log
parsing the log seems significantly easier

File name can be generic because it gets cleaned as part of an accumulation script
i.e. timesheet.log

Everytime a user logs in echo
I, DATE, TIME to the file
A logout or i3lock echos
O, DATE, TIME to the same file

can be dead simple as far as the logging goes
clock -i to clock in
clock -o to clock out

Parser will have to be a bit more robust