This app manipulates dates and times.
Dates are serialized in YYYY-MM-DD format. This has the advantage of
being human-readable and debug-friendly.
Under a given date, a list of time pairs is stored. This is an
array [timePair0, ..., timePairN].
Each time pair is another length-2 array
[checkInTime, checkOutTime]. Both the check-in and the check-out
time are decimal numbers corresponding of 24-hour time events,
e.g. 8.5 for 8:30 AM. Representing time events as numbers facilitates
simple calculation of work hours via subtraction over a time pair and accumulated
daily or yearly work hours via addition.