# Calendar Free/Busy Overlap Paste two .ics calendars and list the time slots where both are free — working hours, minimum meeting length, timezones and DST handled. Free and in-browser. ## Run it - **CLI:** `gizza tool calendar-freebusy-overlap "BEGIN:VCALENDAR VERSION:2.0 BEGIN:VEVENT DTSTART:20260720T090000Z DTEND:20260720T100000Z SUMMARY:Standup END:VEVENT END:VCALENDAR" 'calendar_b=BEGIN:VCALENDAR VERSION:2.0 BEGIN:VEVENT DTSTART:20260720T120000Z DTEND:20260720T130000Z SUMMARY:Lunch sync END:VEVENT END:VCALENDAR'` - **Web:** https://gizza.ai/tools/calendar-freebusy-overlap/ - **Agents:** machine-readable descriptor (parameters JSON Schema) at https://gizza.ai/tools/calendar-freebusy-overlap/tool.json ## Inputs - `calendar_a` — Calendar A (.ics text) _(field)_ - `calendar_b` — Calendar B (.ics text) _(field)_ - `start_date` — First day to scan _(field)_ - `days` — Days to scan _(field)_ - `day_start` — Earliest time (working hours start) _(field)_ - `day_end` — Latest time (working hours end) _(field)_ - `min_minutes` — Minimum slot length (minutes) _(field)_ - `timezone` — Timezone (IANA name) _(field)_ - `weekends` — Include weekends _(field)_ - `output` — Output format _(field)_ ## Output - Common free slots (text) ## Query parameters Open the tool pre-filled and auto-run via URL: - `calendar_a` — Calendar A (.ics text) - `calendar_b` — Calendar B (.ics text) - `start_date` — First day to scan - `days` — Days to scan - `day_start` — Earliest time (working hours start) - `day_end` — Latest time (working hours end) - `min_minutes` — Minimum slot length (minutes) - `timezone` — Timezone (IANA name) - `weekends` — Include weekends - `output` — Output format Example: `https://gizza.ai/tools/calendar-freebusy-overlap/?calendar_a=BEGIN%3AVCALENDAR%0AVERSION%3A2.0%0ABEGIN%3AVEVENT%0ADTSTART%3A20260720T090000Z%0ADTEND%3A20260720T100000Z%0ASUMMARY%3AStandup%0AEND%3AVEVENT%0AEND%3AVCALENDAR&calendar_b=BEGIN%3AVCALENDAR%0AVERSION%3A2.0%0ABEGIN%3AVEVENT%0ADTSTART%3A20260720T120000Z%0ADTEND%3A20260720T130000Z%0ASUMMARY%3ALunch%20sync%0AEND%3AVEVENT%0AEND%3AVCALENDAR&start_date=2026-07-20&days=7&day_start=09%3A00&day_end=17%3A00&min_minutes=30&timezone=Europe%2FBerlin&weekends=true&output=text` --- ## Find the free time two calendars share Trying to schedule with someone whose calendar you can't see live? Ask them for an iCalendar export, paste both files above, and this tool lists every window where **both** of you are free — restricted to working hours, on the days you choose, at least as long as the meeting you need. Everything runs locally in your browser: the calendars never leave your machine. It reads standard **.ics exports** from Google Calendar (Settings → Import & export → Export), Outlook (Save calendar), Apple Calendar (File → Export), Fastmail, Proton and any other RFC 5545 producer. Busy time is taken from `VEVENT`s — including recurring events — and from `VFREEBUSY` busy periods, so an availability-only export works too. Events marked cancelled or free (`STATUS:CANCELLED`, `TRANSP:TRANSPARENT`) don't block time. ## Worked example Calendar A has a standup on Monday 2026-07-20 from 09:00–10:00 UTC; calendar B has a lunch sync from 12:00–13:00 UTC. Scanning that single day between 09:00 and 17:00 with a 30-minute minimum returns: ``` Common free time — 2026-07-20 to 2026-07-20 · Mon–Fri 09:00–17:00 · UTC · ≥30m Calendar A: 1 busy interval · Calendar B: 1 busy interval Mon 2026-07-20 10:00–12:00 (2h) Mon 2026-07-20 13:00–17:00 (4h) 2 free slots · 6h total ``` Pick the **JSON** output for machine-readable slots with ISO 8601 timestamps, or the **iCalendar (.ics)** output to get the shared availability back as an RFC 5545 `VFREEBUSY` calendar (one `FREEBUSY;FBTYPE=FREE` period per slot) that other tools can import. ## What's supported, and the limits - **Recurring events:** `RRULE` with `FREQ=DAILY/WEEKLY/MONTHLY/YEARLY`, `INTERVAL`, `COUNT`, `UNTIL`, weekly `BYDAY` lists (e.g. `MO,WE,FR`), monthly ordinal `BYDAY` (e.g. `3MO` = third Monday), a single `BYMONTHDAY`, and `EXDATE` exceptions. Rules outside this subset (e.g. `BYSETPOS`, hourly frequencies) block only their first occurrence, and the result says so in a note. - **Timezones:** event `TZID`s are resolved against the full IANA database (DST-correct), and the most common Windows/Outlook zone names ("Eastern Standard Time", "W. Europe Standard Time", …) are mapped automatically. Unknown TZIDs fall back to your selected timezone, with a note in the result. - **Moved instances:** a rescheduled occurrence of a recurring meeting (`RECURRENCE-ID`) counts as extra busy time while the original slot also stays blocked — deliberately conservative, so a suggested slot is never secretly busy. - **Caps:** each calendar up to 1 MiB of text; the scan range is 1–60 days; the minimum slot length is 5–720 minutes; up to 20,000 busy intervals after recurrence expansion. - All-day events block the whole day; multi-day events block every day they span. ## FAQ
How do I get the .ics text of a calendar? In Google Calendar open Settings → Import & export → Export, unzip the download and open the `.ics` file in any text editor. In Outlook use File → Save Calendar (choose the date range and full details or free/busy). On macOS/iOS Calendar use File → Export → Export. Then copy the file's text and paste it here. If someone shares a `.ifb` free/busy file, that works too — `VFREEBUSY` periods are understood.
Are recurring meetings like a weekly standup handled? Yes. Daily, weekly, monthly and yearly `RRULE`s are expanded inside the scan range, including `INTERVAL` (every 2 weeks), `COUNT`/`UNTIL` bounds, weekly `BYDAY` day lists, monthly patterns like "third Monday" (`BYDAY=3MO`) and cancelled single occurrences (`EXDATE`). Exotic rules (`BYSETPOS`, `BYWEEKNO`, hourly repeats) are outside the supported subset: such an event blocks only its first occurrence and the result appends a note, so nothing is dropped silently.
What if the two calendars are in different timezones? That's fine — each event carries its own timezone (`TZID` or UTC `Z` times), and every event is converted to the timezone you select before computing overlap, with daylight saving handled by the IANA database. The working-hours window and the result times are expressed in your selected timezone. Common Windows zone names used by Outlook exports are mapped to IANA zones automatically; an unknown TZID falls back to your selected zone and the result tells you.
Why is a slot shown when one calendar looks busy (or the other way around)? Check three things. First, events marked free (`TRANSP:TRANSPARENT`, how "Free" shows in Outlook) and cancelled events deliberately don't block time. Second, make sure the selected timezone is the one you expect — the slot list is expressed in it, not in the calendar's zone. Third, a gap shorter than the minimum slot length is hidden; lower "Minimum slot length" to see it. In the other direction, a moved instance of a recurring meeting keeps its original slot blocked too (conservative on purpose).
Is my calendar data uploaded anywhere? No. The page is a static site running a WebAssembly module; parsing and the overlap computation happen entirely in your browser. Your `.ics` text is never sent to a server, which is exactly why this tool asks you to paste the export instead of a calendar URL.
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