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1. The "DTSTART" property SHOULD be synchronized with the recurrence rule, if specified.

2. The "RRULE" property SHOULD NOT occur more than once in a component.

3. The BYHOUR, BYMINUTE, and BYSECOND rule parts MUST NOT be specified in the "RRULE" property when the "DTSTART" property is specified as a DATE value.

4. The value type of the "DTEND" or "DUE" properties MUST match the value type of "DTSTART" property.

5. The "DURATION" property can no longer appear in "VFREEBUSY" components.

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