ICS Timezone Shifter
Paste an iCalendar file and rewrite every timed event into a new timezone. Preserve instants, repair mislabeled exports, keep all-day events fixed, and choose TZID, UTC, or floating output.
About this tool
Calendar exports are easy to get almost right and still be unusable. A meeting written as DTSTART:20240710T090000 means "9 AM somewhere" until the importer decides what timezone "somewhere" is. A meeting written as DTSTART;TZID=America/New_York:20240710T090000 has a zone, but it may need to become Berlin time for another system. This tool rewrites the timed parts of an iCalendar file so the result imports with the timezone model you choose.
A default conversion preserves the instant in time and expresses it in the target timezone. For example:
BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
UID:demo@example
DTSTART:20240310T140000Z
DTEND:20240310T150000Z
SUMMARY:Standup
END:VEVENT
END:VCALENDAR
with target Europe/Berlin becomes a calendar with a generated VTIMEZONE and event times like:
DTSTART;TZID=Europe/Berlin:20240310T150000
DTEND;TZID=Europe/Berlin:20240310T160000
Use Source timezone for floating times when the input values have no Z and no TZID. Existing TZID parameters in the file are trusted in convert mode. Use Relabel mode when the wall-clock digits are already correct but the zone marker is wrong: 09:00Z can become 09:00 America/New_York rather than the equivalent UTC instant.
Write output times as controls the format of the result. TZID writes target-zone local values and can include a fresh VTIMEZONE. UTC writes ...Z values and omits timezone definitions. Floating writes local wall-clock values with no zone marker, for importers that assign the zone outside the file.
Limits and edge cases:
- Up to 5,000
VEVENTblocks per run. - Timed
DTSTART,DTEND,DUE,RECURRENCE-ID,EXDATE,RDATE, andRRULEUNTILvalues are rewritten. - All-day
VALUE=DATEevents never move. DTSTAMP,CREATED,LAST-MODIFIED, alarms, attendees, descriptions, summaries, and unknown properties pass through.- Existing
VTIMEZONEblocks are removed before an optional fresh target-zone block is inserted. - Ambiguous fall-back times use the earlier occurrence; spring-forward gap times roll forward one hour.
- Lines are unfolded before processing and folded back to 75-octet iCalendar lines.
FAQ
What is the difference between convert and relabel?
Convert preserves the real instant. If a meeting is 20240310T140000Z, Berlin output is 20240310T150000 because 14:00 UTC is 15:00 in Berlin on that date. Relabel preserves the written clock digits and changes what timezone they mean. Use relabel only when an export says the wrong timezone but the local meeting time is already what you want.
Will all-day events shift to the previous or next day?
No. All-day iCalendar values are written as VALUE=DATE, not as instants. The tool leaves those values unchanged, so a holiday or vacation day does not slide across a date boundary just because the target timezone is far away.
Why does the tool replace VTIMEZONE blocks?
Once every timed value is rewritten into the target zone, the old timezone definitions are stale. The tool removes them and, when writing TZID output, inserts one target-zone VTIMEZONE built from the IANA timezone data used by chrono-tz. If your importer already has its own timezone database, turn off the generated block.
Does this modify recurrence rules?
It keeps recurrence rules intact and only rewrites an UNTIL date-time when one is present. Date-only UNTIL values are left alone. Exception date lists (EXDATE) and recurrence date lists (RDATE) are shifted along with event starts and ends.
Can I paste only a VEVENT instead of a full VCALENDAR?
Yes. Bare event blocks are accepted and wrapped in a minimal VCALENDAR so the output imports cleanly. A full .ics file is preferred when you want calendar-level fields such as VERSION, PRODID, or X-WR-TIMEZONE preserved.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool ics-timezone-shifter "BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
UID:demo@example
DTSTART:20240310T140000Z
DTEND:20240310T150000Z
SUMMARY:Standup
END:VEVENT
END:VCALENDAR"New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/ics-timezone-shifter/?input=BEGIN%3AVCALENDAR%0AVERSION%3A2.0%0ABEGIN%3AVEVENT%0AUID%3Ademo%40example%0ADTSTART%3A20240310T140000Z%0ADTEND%3A20240310T150000Z%0ASUMMARY%3AStandup%0AEND%3AVEVENT%0AEND%3AVCALENDAR&from=America%2FNew_York&to=Europe%2FBerlin&mode=convert&write_as=tzid&include_vtimezone=trueMachine-readable descriptor: tool.json — title + parameters JSON Schema for agents.