CorrectICS

Fix “Invalid ICS File” Errors (Google & Outlook)

(Google Calendar, Outlook, Apple Calendar, iCloud, Teams, Office 365)

If you’re trying to import an .ics file into Google Calendar, Outlook, Apple Calendar, or a custom app — and it won’t load — you’re not alone.
ICS files follow the iCalendar specification, but real-world exports are often invalid, incomplete, or malformed.

This guide explains:

  • The most common ICS errors

  • Why Google/Outlook/iCloud reject certain files

  • Examples of broken vs corrected ICS snippets

  • How to validate or autofix issues programmatically

  • When to use a tool like CorrectICS to clean the file

This page is optimized for engineers, QA teams, and operators who deal with ICS imports/exports from CRMs, booking systems, and legacy tools.

1. “This event cannot be saved” / “Unable to import file”

This is the #1 ICS failure across Google Calendar, Outlook, and Apple Calendar.

Cause

The ICS is missing one of the required VCALENDAR or VEVENT boundaries:

  • BEGIN:VCALENDAR

  • END:VCALENDAR

  • BEGIN:VEVENT

  • END:VEVENT

Broken Example

BEGIN:VCALENDAR VERSION:2.0 BEGIN:VEVENT DTSTART:20250310T090000Z SUMMARY:Team Meeting

Corrected Example

BEGIN:VCALENDAR VERSION:2.0 BEGIN:VEVENT DTSTART:20250310T090000Z SUMMARY:Team Meeting END:VEVENT END:VCALENDAR

Fix

Ensure all component blocks are properly closed.

Automatic fix: upload file to CorrectICS
Programmatic fix: use the CorrectICS API (see section 9).

2. Missing or invalid DTSTART/DTEND

Most calendar clients require:

  • DTSTART (start time)

  • DTEND or DURATION

Broken Example

BEGIN:VEVENT SUMMARY:Lunch DTEND:20250310T130000Z END:VEVENT

Corrected Example

BEGIN:VEVENT DTSTART:20250310T120000Z DTEND:20250310T130000Z SUMMARY:Lunch END:VEVENT

Google Calendar is especially strict here — if DTSTART is missing, the file fails silently.

3. Invalid or unsupported TIME ZONES (TZID issues)

This is one of the most subtle ICS problems.
Common errors:

  • TZID=EST (not valid — needs Olson format)

  • TZID=GMT-5 (nonstandard)

  • Missing VTIMEZONE block

  • TZID mismatch between DTSTART and VTIMEZONE

Broken Example

DTSTART;TZID=EST:20250310T090000

Google and Apple Calendar do not accept EST as a valid TZID.

Corrected Example

DTSTART;TZID=America/New_York:20250310T090000

If the VTIMEZONE block is missing

Some clients (Outlook desktop) require it for floating times.

Automatic fix

CorrectICS automatically:

  • normalizes TZIDs

  • adds missing VTIMEZONE blocks

  • resolves ambiguous time zones

4. RRULE / Recurrence errors

Recurring events are the most frequent source of ICS failures.

Common issues:

  • Missing FREQ=

  • Invalid BYDAY or BYMONTH

  • Unsupported rules (Outlook Web is stricter than Apple/Google)

  • Infinite recurrences with no UNTIL or COUNT (some clients reject)

Broken Example

RRULE:BYDAY=MO,WE,FR

Corrected Example

RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR

Broken Example 2 (invalid UNTIL date)

RRULE:FREQ=DAILY;UNTIL=20250230T000000Z

Corrected

RRULE:FREQ=DAILY;UNTIL=20250228T000000Z

CorrectICS validates and normalizes recurrence rules so they work in all major clients.

5. Duplicate or missing UID (required by the spec)

Every event must have a globally unique UID.

If you generate ICS files programmatically (from CRMs, custom apps, CSV imports), you may forget to include one.

Broken Example

BEGIN:VEVENT DTSTART:20250310T090000Z SUMMARY:Meeting END:VEVENT

Corrected Example

BEGIN:VEVENT UID:[email protected] DTSTART:20250310T090000Z SUMMARY:Meeting END:VEVENT

CorrectICS auto-generates stable UIDs if missing.

6. Missing VERSION or PRODID

Some calendar apps silently reject ICS files missing:

  • VERSION:2.0

  • PRODID:-//YourApp//EN

These fields seem harmless, but older Outlook and Apple Calendar builds treat them as required.

Broken Example

BEGIN:VCALENDAR BEGIN:VEVENT ...

Corrected Example

BEGIN:VCALENDAR VERSION:2.0 PRODID:-//CalendarExporter 1.0//EN

CorrectICS adds these if missing.

7. CRLF vs LF line endings (Windows vs Unix)

ICS files should use CRLF (\r\n).
Many exporters use just LF (\n), which breaks:

  • older Outlook

  • some Exchange servers

  • certain enterprise calendar sync tools

CorrectICS normalizes line endings automatically.

8. Long lines / folded lines incorrectly formatted

ICS uses a line-folding mechanism:

  • Lines >75 bytes must be folded

  • Continuation lines must begin with a space

Broken Example

DESCRIPTION:This is a very long line that goes over the recommended length which will cause some ICS consumers to reject the file outright or silently fail to parse it

Corrected Example

DESCRIPTION:This is a very long line that goes over the recommended length   that will be folded correctly.

This is annoying to do manually — CorrectICS handles it for you.

9. How to validate & autofix ICS programmatically

Developers often prefer API workflows instead of manual uploads.

Validation

curl -X POST https://api.correctics.com/v1/validate \
   -H "Content-Type: text/calendar" \
   --data-binary @broken.ics

Autofix

curl -X POST https://api.correctics.com/v1/autofix \
   -H "Content-Type: text/calendar" \
   --data-binary @broken.ics \
   -o fixed.ics

Typical uses:

  • nightly CI checks on calendar exports

  • ingesting ICS feeds from vendors

  • verifying ICS generated from CSV → ICS pipelines

  • sanity checking user-uploaded calendar files in a SaaS app

10. When you should NOT manually repair ICS files

If:

  • you’re a SaaS app supporting many users,

  • you import ICS from multiple vendors,

  • you export ICS for other systems, or

  • you deal with recurring time zone issues,

…then manually fixing files is not sustainable.

Use:

  • CorrectICS for one-off fixes

  • CorrectICS API for automation

Your total time spent on ICS goes from hours to ~5 seconds.

11. Conclusion: ICS files break for predictable reasons

Most ICS failures boil down to:

  • missing structural boundaries

  • missing DTSTART/DTEND

  • invalid time zones

  • malformed RRULEs

  • missing VERSION/PRODID/UID

  • bad line endings

  • invalid line folding

If you’re debugging ICS files regularly — or supporting customers who are — the fastest way out is to:

  • validate with CorrectICS

  • autofix automatically

  • integrate the API into your testing or ingestion flow

Fix your .ics file in seconds

Upload an iCalendar file and get a clean, import-ready version for Google Calendar, Outlook, Apple Calendar, and Teams.

Try the CorrectICS Autofix Tool Back to Help