Using API v1? See what changes in v2, or the v1 docs

VALIDATION_INVALID_TIMEZONE

The time zone is not an IANA time zone name.

HTTP status: 400. Retrying the same request returns the same error.

What to do

Send an IANA time zone name, for example America/New_York or Europe/London.

Hint returned with the error: Use an IANA time zone name such as America/New_York, not an abbreviation or an offset.

Why it happens

An abbreviation such as EST
A Windows time zone name such as Eastern Standard Time
An offset such as -05:00

Endpoints that return it

Create a calendar POST /v2/calendars
Update a calendar PATCH /v2/calendars/{calendarRef}
Create an event POST /v2/events
Update an event PATCH /v2/events/{eventId}
Create an event from text POST /v2/events/quick

VALIDATION_INVALID_QUERY