One file every calendar understands
The .ics format, defined by the iCalendar standard (RFC 5545), is the closest thing calendars have to a universal language. Google Calendar, Apple Calendar, Outlook, Thunderbird and almost every phone calendar can open one, which makes it the reliable way to say "put this in your diary" without knowing what software the other person uses. Fill in the event above and the file contents update live; download the .ics and open it, attach it to an email, or link to it from a website's "add to calendar" button.
Inside, the file is plain text with strict rules: every line ends with a carriage return and line feed, commas and semicolons in your text are escaped, and long lines are folded so no line exceeds 75 bytes. The tool follows those rules exactly, which is what keeps fussy older calendar apps happy alongside the modern ones.
How times and time zones work
Timed events are stored in UTC, the global reference time, and every calendar app converts them back to each viewer's local time. You enter times in your device's local time zone, so create the file on a device set to the zone where the event happens and everyone sees the right moment wherever they are.
All-day events work differently: they are stored as dates with no time at all, so a birthday stays on the same date in every time zone instead of drifting overnight for readers far away. One quirk of the standard is that the end date is exclusive, meaning a one day event on 12 August is stored as ending on 13 August. The tool applies that rule automatically, so you simply enter the last day of the event.
Sharing the event
The simplest route is attaching the .ics to an email; recipients tap the attachment and their calendar offers to add it. On a website, upload the file and link to it from an "add to calendar" button. Because each copy of the file carries a unique identifier, opening it twice updates the same event rather than creating duplicates in most calendar apps.
For recurring invitations, newsletters and event pages, generate one file per event and give each a clear name; the tool names the download after your event title. If you need a countdown to build anticipation for the same event, the countdown to a date tool pairs nicely, and the working days calculator will tell you how many working days remain before it.
Built in your browser, never uploaded
Event details can be sensitive, from medical appointments to unannounced product launches, so the file is assembled entirely on your device by JavaScript. Nothing you type is transmitted, logged or stored; the download comes straight from your browser's memory. Close the tab and everything is gone.
That also means you can generate as many variations as you like, instantly. Check the live file contents box if you are curious about the format itself; it is a nice little example of a 1990s internet standard still quietly running everyone's diary.
Frequently asked questions
What is a .ics file?
It is the standard calendar file format, defined by an internet standard called iCalendar (RFC 5545). Nearly every calendar app understands it, so one file works for Google Calendar, Apple Calendar, Outlook and most others. Opening the file offers to add the event to your calendar.
Is anything uploaded when I create the file?
No. The file is assembled by JavaScript running on your own device and offered straight back as a download. Event details never travel over the internet, nothing is logged or stored, and closing the tab removes everything.
How are time zones handled?
The times you enter are read in your device's local time zone and stored in the file as UTC, which every calendar app converts back to each viewer's local time. Enter 2pm UK time and a recipient in New York sees 9am, which is the same moment. Create the file on a device set to the event's time zone.
What does the all-day option do?
It stores the event as a date rather than a time, so it appears in the all-day band at the top of the calendar and stays on the right date in every time zone. Behind the scenes the end date is stored as the day after the event finishes, because the format treats end dates as exclusive; the tool handles that for you.
How do I share the event with other people?
Send them the .ics file by email or chat, or link to it from a website. Each person opens it and their own calendar app adds the event. For a large mailing list, attach the file to the invitation email so guests can add it with one tap.