--- title: "ICS Diff" description: "Compare two .ics exports of the same calendar and see which events were added, removed or changed, matched by UID, with the before and after value of every changed field. Download the report and the added or removed events as calendars of their own. On this page, both files are read in your browser and nothing is uploaded." url: https://findutils.com/convert/ics-diff/ category: convert --- # ICS Diff Compare two .ics exports of the same calendar and see which events were added, removed or changed, matched by UID, with the before and after value of every changed field. Download the report and the added or removed events as calendars of their own. On this page, both files are read in your browser and nothing is uploaded. **Use this tool:** [ICS Diff](https://findutils.com/convert/ics-diff/) ## Programmatic access - REST id `ics-diff`: POST https://api.findutils.com/api/tools/ics-diff/execute (reference: https://findutils.com/api/ics-diff/) - MCP tool `ics_diff` on https://mcp.findutils.com (reference: https://findutils.com/mcp/ics-diff/) - Data: The REST API and the MCP tool are a separate path from this page: a call sends the input you include to FindUtils over HTTPS, where it is processed and the result returned. FindUtils does not store that input, and logs the request without its content. ## Why Compare Two Calendar Exports? After a migration, a sync conflict or a re-export, the question is always the same: what actually changed? Opening two .ics files in a text editor does not answer it, because long lines are folded at 75 characters, properties come in any order, and every export rewrites stamps such as DTSTAMP. Two calendar apps side by side do not answer it either once there are more than a handful of events. This page matches the events of both files by their UID, the identifier calendar apps use for the same purpose, and lists exactly which were added, which were removed, and which fields of the rest changed. ## Comparison Tips - Put the older file on Before (A). Added means only in After, removed means only in Before. - A modified occurrence of a recurring meeting has the series UID plus a RECURRENCE-ID, so it is compared as its own entry, never against the series. - An event without a UID cannot be matched safely, so it is listed apart instead of being guessed. - Turn off the DTSTAMP, LAST-MODIFIED and SEQUENCE option to see every stamp a calendar app rewrote. - Times are compared as written: the same meeting stated in UTC in one file and in Europe/Istanbul in the other shows as a DTSTART change. ## Frequently Asked Questions ### How does ICS Diff match events between the two files? By UID together with RECURRENCE-ID. The UID is the identifier a calendar app gives each event and keeps when the event is edited, so the same event has the same UID in both exports even when its title, time or place changed. A modified occurrence of a recurring event shares the series UID and adds a RECURRENCE-ID, so it is matched as its own entry. Events are never matched by title or position. ### What counts as a changed event? An event that is in both files and differs in at least one field: a property such as SUMMARY, DTSTART, DTEND, LOCATION, DESCRIPTION, RRULE, STATUS or ATTENDEE, or an alarm (VALARM). Each changed event lists the fields with their before and after values. Property order, line folding and parameter order are not changes, and DTSTAMP, LAST-MODIFIED and SEQUENCE are ignored by default. ### Why are DTSTAMP, LAST-MODIFIED and SEQUENCE ignored? Because calendar apps rewrite them without anyone editing the event. DTSTAMP is usually the moment the file was exported, so it differs on every event between two exports, and LAST-MODIFIED and SEQUENCE move with sync activity. Counting them would mark almost every event as changed. Turn the option off to compare them too. ### What happens to events without a UID? They are listed under the table by side, with their title and start, and are not counted as added, removed or changed. Matching them by title or time would be a guess that can pair two different events, so the page does not try. ### How are recurring events compared? A recurring series is one entry, compared by its fields like any other, so a changed rule shows as a change to RRULE and a new exception date as a change to EXDATE. Each modified occurrence is a separate entry with its own RECURRENCE-ID. The rules are not expanded into occurrences; to compare the actual dates, run both files through [ICS Expand RRULE](https://findutils.com/convert/ics-expand-rrule/) with the same date range first. ### Are time zones converted before comparing? No. Times are compared as written, with their TZID or trailing Z. The same meeting written as 06:30 UTC in one file and as 09:30 Europe/Istanbul in the other shows as a DTSTART change, which is what changed in the file. The page does not decide that the two are the same instant. ### What are added.ics and removed.ics? They are the added and the removed events as calendars of their own, each a complete VCALENDAR with the header of its source file and the VTIMEZONE blocks its events use. Import removed.ics to bring deleted events back, or added.ics to copy just the new events into another calendar. Changed events stay in the report; no merged calendar is written. ### What is in the JSON report? The two file names, the match key, the ignored fields, the four counts, and one entry per difference with change (added, removed or changed), type, uid, recurrence_id, summary, start and, for a changed event, each field with its before and after values. Entries without a UID and any notes are included too. The table on the page shows up to 300 rows; the report holds every one. ### Is my calendar uploaded? Not on this page. Both files are read with the browser's File API and compared in the tab, and the page makes no request with the calendar text, which you can confirm in the browser's network panel. The REST API and MCP tool are a separate path: a call sends the input you include to FindUtils over HTTPS, where it is processed and the result returned. FindUtils does not store that input, and logs the request without its content. ### Can I compare calendars from a script? Yes. The same code runs on the FindUtils REST API and MCP server as the ics_diff tool. Send the two .ics texts as before and after, optionally with ignore_metadata and include_todos, and read the counts, the differences with their fields, the entries without a UID, and added_ics and removed_ics from the result. ## Related Tools - [Merge ICS](https://findutils.com/convert/merge-ics/) - [ICS to JSON](https://findutils.com/convert/ics-to-json/) - [ICS Expand RRULE](https://findutils.com/convert/ics-expand-rrule/) - [ICS Filter](https://findutils.com/convert/ics-filter/) - [ICS to CSV](https://findutils.com/convert/ics-to-csv/) - [Split ICS](https://findutils.com/convert/split-ics/) - [CSV Diff](https://findutils.com/convert/csv-diff/)