Add helper subroutines for converting UNIX timestamps to DateTime objects and string dates
If "INVALID_ARG_UNIT_REFERENCE_TS" error code is returned, print a custom error message with a erroneous timestamp + string date
Check if start_timestamp doesn't exceed end_timestamp
When fetching start_date and end_date, check either of the timestamps exceed current date; if they do, reset the start_date / end_date to current timestamp (because Bit.ly API will complain about timestamps in the future with the "INVALID_ARG_UNIT_REFERENCE_TS" error)