Change some variable names and add PHPDoc for class variables.
Add a better function to get next cron run time
Set the run-time array from new function
Set isExecutable() to use calculated times
Rename cron interval names to use the event class constants for
conssitency
Simplyfy logic in isExecutable() as we now use consistent names
Cope with old style interval names
Missed removal of class variable
Only use text date on db write
Handle db updates for old style cron records
PSR-2 clean ups