How do I create a link to add an entry to a calendar?

The links in Dave’s post are great. Just to put a few technical details about the google links into an answer here on SO:

Google Calendar Link

<a href="http://www.google.com/calendar/event?action=TEMPLATE&text=Example%20Event&dates=20131124T010000Z/20131124T020000Z&details=Event%20Details%20Here&location=123%20Main%20St%2C%20Example%2C%20NY">Add to gCal</a>

the parameters being:

• action=TEMPLATE (required)
• text (url encoded name of the event)
• dates (ISO date format, startdate/enddate – must have both start and end time – the button generator will let you leave the endtime blank, but you must have one or it won’t work.)
  • to use the user’s timezone: 20131208T160000/20131208T180000
  • to use global time, convert to UTC, then use 20131208T160000Z/20131208T180000Z
  • all day events, you can use 20131208/20131209 – note that the button generator gets it wrong. You must use the following date as the end date for a one day all day event, or +1 day to whatever you want the end date to be.
• details (url encoded event description/details)
• location (url encoded location of the event – make sure it’s an address google maps can read easily)

Update Feb 2018:

Here’s a new link structure that seems to support the new google version of google calendar w/o requiring API interaction:

https://calendar.google.com/calendar/r/eventedit?text=My+Custom+Event&dates=20180512T230000Z/20180513T030000Z&details=For+details,+link+here:+https://example.com/tickets-43251101208&location=Garage+Boston+-+20+Linden+Street+-+Allston,+MA+02134

New base url: https://calendar.google.com/calendar/r/eventedit

New parameters:

• text (name of the event)
• dates (ISO date format, startdate/enddate – must have both start and end time)
  • an event w/ start/end times: 20131208T160000/20131208T180000
  • all day events, you can use 20131208/20131209 – end date must be +1 day to whatever you want the end date to be.
• ctz (timezone such as America/New_York – leave blank to use the user’s default timezone. Highly recommended to include this in almost all situations. For example, a reminder for a video conference: if three people in different timezones clicked this link and set a reminder for their “own” Tuesday at 10:00am, this would not work out well.)
• details (url encoded event description/details)
• location (url encoded location of the event – make sure it’s an address google maps can read easily)
• add (comma separated list of emails – adds guests to your new event)

Notes:

• the old url structure above now redirects here
• supports https
• deals w/ timezones better
• accepts + for space in addition to %20 (urlencode vs rawurlencode in php – both work)