ICS (.ics) Parser — Inspect iCalendar events, todos, and recurrence

The business-context density inside a calendar file

At first glance an .ics file looks like a list of dates. But the ORGANIZER, ATTENDEE, LOCATION, and DESCRIPTION fields pack in the full context of every meeting: who is involved, where it takes place, and what the agenda says. The ATTENDEE list is a direct export of your professional contact graph — email addresses of clients, colleagues, and candidates. SUMMARY lines often expose project names, deal stages, and recruitment pipeline steps.

Recurring rules (RRULE) reveal standing meetings — weekly standups, monthly reviews. All-day VALUE=DATE spans can map out leave periods or travel schedules. A .ics exported from Google Calendar or Outlook is enough to reconstruct a near-complete picture of someone’s working life.

Why uploading to an online iCalendar viewer is a privacy concern

When you upload a .ics to an online parser or viewer, the full ATTENDEE roster, DESCRIPTION notes, LOCATION strings (including internal facility names), and any confidential meeting details arrive at the operator’s server. Many of these tools sit in ambiguous territory in their terms of service about what they do with uploaded content.

The typical use case — checking a meeting invite received from a client, or inspecting a corporate calendar export before importing it — is exactly the scenario where the file contains sensitive information. The file lands on a third-party server the moment you click Upload, before you even know what’s in it.

How the custom RFC 5545 parser stays inside your browser

This tool runs a hand-built iCalendar 2.0 parser (RFC 5545 + RFC 7986 extensions) entirely in browser JavaScript. It handles line folding (unfolding continuation lines that begin with a space or tab), component nesting (VCALENDAR > VEVENT > VALARM), DATE-TIME reformatting from basic to extended form, and escape-sequence processing — all without any external API call.

ATTENDEE email addresses, DESCRIPTION text, and vendor-specific X- fields all stay in page memory. DevTools Network shows no outgoing requests after the initial page load. The source is on GitHub so the parsing logic is auditable.

Reviewing before forwarding or importing

Before forwarding a meeting invite to an external party, or before importing calendar data into another system, use this tool to confirm what ATTENDEE addresses, DESCRIPTION content, and LOCATION strings are actually in the file. It is common for calendar software to auto-populate DESCRIPTION with previous notes, agenda templates, or Zoom links that you may not want to send to a new audience. Inspect the parsed output, edit the raw .ics if needed, and then forward or import with confidence. For related contact-card or e-book metadata reviews, vcard-info and epub-info work the same way.

The RFC 5545 record structure and line folding

iCalendar is a line-oriented ASCII format. Each line is PROPERTY-NAME[;PARAM=VALUE]:VALUE\r\n. VCALENDAR sits at the top, with VEVENT, VTODO, VJOURNAL, VFREEBUSY, and VTIMEZONE nested inside. RFC 5545 requires that lines longer than 75 octets be folded by inserting CRLF followed by a space or tab; the parser must reverse that folding before splitting on property boundaries. This tool unfolds first and then walks property by property.

DATE-TIME values come in three flavours. 20260610T093000Z (trailing Z) is UTC. 20260610T093000 without Z is floating time, meaning the moment is interpreted in the local timezone of whoever reads it. TZID=Asia/Tokyo:20260610T093000 is anchored to a named timezone. VTIMEZONE components carry their own timezone definitions through STANDARD and DAYLIGHT sub-components that describe daylight-saving transitions. Olson names like Asia/Tokyo are standard, but Microsoft Outlook emits its own labels such as Tokyo Standard Time — interoperating across both requires a mapping table.

RRULE / EXDATE / RDATE recurrence and RFC 7986 extensions

RRULE is a one-line DSL for recurring events: RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR;UNTIL=20261231T000000Z. FREQ is one of SECONDLY / MINUTELY / HOURLY / DAILY / WEEKLY / MONTHLY / YEARLY, modified by INTERVAL, COUNT, UNTIL, BYDAY, BYMONTH, BYSETPOS, and others. Complex rules — FREQ=MONTHLY;BYDAY=2TU,4TU for “second and fourth Tuesday” — are not always interpreted identically across Google Calendar, iCloud, and Outlook, which is a known source of interoperability bugs.

RFC 7986 (2016) added NAME, REFRESH-INTERVAL, SOURCE, COLOR, IMAGE, and CONFERENCE properties to iCalendar. Zoom and Microsoft Teams URLs land in CONFERENCE or in vendor-specific X-* properties such as X-MS-OLK-CONFTYPE. This tool surfaces X- properties verbatim so you can spot vendor-specific data that may not need to travel with a forwarded invite. The METHOD property (PUBLISH, REQUEST, REPLY, CANCEL, COUNTER) classifies the file’s purpose; understanding it before re-distributing helps decide whether the recipient should be treated as an attendee or a new audience.