What does the validator check?

It verifies that BEGIN and END tags are balanced and correctly nested, that the file begins with VCALENDAR and declares VERSION and PRODID, and that each VEVENT, VTODO, and VTIMEZONE has the properties RFC 5545 requires, such as UID and DTSTAMP.

Does a passing file mean my event is correct?

It means the file is structurally valid and has the mandatory properties. It cannot judge whether your dates or recurrence rules describe what you intended, only that they are well-formed.

What date formats are accepted?

DATE values like 20260610, UTC date-times ending in Z like 20260610T090000Z, and local or TZID date-times. The validator flags strings that do not match any of these patterns.

Why does it warn about line endings or folding?

RFC 5545 requires CRLF line breaks and folds long lines at 75 octets with a leading space. Files using bare newlines still parse in most apps, so these are reported as warnings rather than hard errors.

Is my calendar uploaded?

No. The whole file is parsed in your browser, so the events, attendees, and notes inside it stay completely private.

ICS / iCal Validator

An ICS validator checks that an iCalendar file follows RFC 5545 before you rely on it. A malformed .ics may import silently wrong, drop events, or be rejected outright by strict apps. This free tool parses the file structure and the required properties of each component, then lists exactly what is missing or malformed — all in your browser.

How it works

The validator unfolds the file (joining continuation lines that begin with a space or tab), then walks it line by line:

Structure. It maintains a stack of BEGIN: components and matches each END:. Mismatched or unclosed components are errors, and the outermost component must be VCALENDAR.
Calendar properties. VERSION and PRODID are required directly inside VCALENDAR.
Component requirements. A VEVENT needs UID and DTSTAMP; a VTODO needs the same; a VTIMEZONE needs TZID and at least one STANDARD or DAYLIGHT sub-component.
Value checks. Date and date-time values are tested against the allowed patterns, and lines without a property name before the colon are flagged.

Each issue is reported with the line number and component so you can jump straight to it. Warnings cover non-fatal style issues like bare-newline line endings.

Tips and notes

A file can be valid yet still describe the wrong event — the validator only checks form, not intent.
If you exported from a tool that used plain newlines, expect a line-ending warning; most calendars accept it anyway.
The most common real error is a missing UID, which makes calendars duplicate the event on every re-import.
Pair this with the ICS builder to generate compliant files from scratch.