Technical users often ask how to format an iCal file so that an event stays at the same time regardless of where the user is. (e.g., “Wake up at 7:00 AM” regardless of the city).

The iCal standard supports two distinct ways to specify event times: floating time and fixed time, and choosing the right one significantly affects how your event behaves across different time zones.

Floating Time (Local Time)

Floating time is when an event is defined without any timezone information. It is interpreted as the local time of whichever device or calendar app opens the file.

Use floating time for events that should always happen at the same clock time, regardless of the observer’s location. Examples include:

• Daily alarms (e.g., “Wake up at 7:00 AM”)
• Recurring daily tasks, e.g. a medication reminder that follows the patient’s local clock
• Events that are tied to a specific local time in a specific place, but you want them to “move” with the user.

iCal Example

Omit any timezone identifier and the trailing Z from DTSTART and DTEND.

BEGIN:VEVENT DTSTART:20260413T070000 DTEND:20260413T080000 SUMMARY:Daily Alarm END:VEVENT 

In this example, if you are in London, it’s 7:00 AM. If you travel to New York, it’s still 7:00 AM local time.

Fixed Time (Absolute Time)

Fixed time uses a specific timezone offset (like Z for UTC) or a TZID parameter to anchor the event to a specific moment in global time.

Use fixed time for events that happen at a single, unambiguous moment in history, regardless of where the participants are located. Examples include:

• Webinars or online meetings
• Flight departures
• Concerts or scheduled broadcasts
• A recurring event where daylight saving adjustments should apply automatically

iCal Example: UTC (Universal Coordinated Time)

Using the Z suffix indicates the time is in UTC (Coordinated Universal Time). Calendar apps will convert it to the viewer’s local time automatically.

BEGIN:VEVENT DTSTART:20260413T120000Z DTEND:20260413T130000Z SUMMARY:Global Webinar END:VEVENT 

iCal Example: Specific Timezone (TZID)

Using TZID pins the event to a specific named timezone (e.g., America/New_York). This is the most explicit and portable form identifying a precise moment in time.

BEGIN:VEVENT DTSTART;TZID=America/New_York:20260413T070000 DTEND;TZID=America/New_York:20260413T080000 SUMMARY:New York Morning Meeting END:VEVENT 

In this example, even if you view the event in Tokyo, the calendar will calculate the correct local time for you based on the New York offset.