May be one of:
- Frequency::SECONDLY to specify repeating events based on an
interval of a second or more.
- Frequency::MINUTELY to specify repeating events based on an
interval of a minute or more.
- Frequency::HOURLY to specify repeating events based on an
interval of an hour or more.
- Frequency::DAILY to specify repeating events based on an
interval of a day or more.
- Frequency::WEEKLY to specify repeating events based on an
interval of a week or more.
- Frequency::MONTHLY to specify repeating events based on an
interval of a month or more.
- Frequency::YEAR to specify repeating events based on an
interval of a year or more.