The following iCalendar snippet works in Fantastical and Calendar.app
BEGIN:VCALENDAR
BEGIN:VEVENT
SUMMARY:Test Event
DTSTART;TZID=America/Los_Angeles:20160630T120000
DTEND;TZID=America/Los_Angeles:20160630T180000
END:VEVENT
END:VCALENDAR
Quotes around the TZID breaks it in both iCalendar and Fantastical.
BEGIN:VCALENDAR
BEGIN:VEVENT
SUMMARY:Test Event
DTSTART;TZID="America/Los_Angeles":20160630T120000
DTEND;TZID="America/Los_Angeles":20160630T180000
END:VEVENT
END:VCALENDAR