recurrence property
An RRULE with format [RFC-5545](https://tools.ietf.org/html/rfc5545#section-3.8.5.3) for how this window reccurs.
They go on for the span of time between the start and end time. The only
supported FREQ value is "WEEKLY". To have something repeat every weekday,
use: "FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR". This specifies how frequently the
window starts. To have a 9 am - 5 pm UTC-4 window every weekday, use
something like: start time = 2019-01-01T09:00:00-0400 end time = 2019-01-01T17:00:00-0400 recurrence = FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
Required.
Implementation
core.String? recurrence;