Specific time specification.
A Specific interval is more complex, but defines a certain time to run and
the days that it should run. It has the following attributes:
time - the time of day to run, as 'HH:MM'
ordinals - first, second, third &c, as a set of integers in 1..5
months - the months that this should run, as a set of integers in 1..12
weekdays - the days of the week that this should run, as a set of integers,
0=Sunday, 6=Saturday
timezone - the optional timezone as a string for this specification.
Defaults to UTC - valid entries are things like Australia/Victoria
or PST8PDT.
A specific time schedule can be quite complex. A schedule could look like
this:
'1st,third sat,sun of jan,feb,mar 09:15'
In this case, ordinals would be {1,3}, weekdays {0,6}, months {1,2,3} and
time would be '09:15'.