5.1.6. AVAILABILITY
DTSTARTandDTENDif specifiedMUSTbe date-time values.DTSTART MUSTappear once and signifies start of the free period.Only one of
DTENDorDURATION MAYappear and signify the end of the free period.
AVAILABILITYDTSTART and DTEND if specified MUST be date-time values.
DTSTART MUST appear once and signifies start of the free period.
Only one of DTEND or DURATION MAY appear and signify the end of the free period.