calendargoogle-calendar
Google Calendar suggests that 42 of my friends have their birthdays on January 1st. Is it an error about how Google collects birthday information? There is just no way that so many people are born on that day. (Below is an image of the birthdays shown in month mode by Google Calendar, with names hidden for privacy reasons)
Google says every few hours, which in my experience means 6-24.
Currently, calendar feeds are automatically updated by Google Calendar every few hours. We understand that some users want to have the ability to refresh the calendars they added manually and we are exploring various ways to enable this.
If the calendar you're subscribing to using Google Calendar isn't refreshed after 24 hours, please send us the calendar address and the detail of the problem so that we can investigate.
Although Google Calendar doesn't allow you to create or modify arbitrary recurrence rules, the iCalendar specification which it uses and understands is quite robust in this regard.
Create an ICS file with a custom recurrence rule (RRULE). For example, for an all-day event on the first, third, and fifth Fridays of every month, starting 2 March 2012:
BEGIN:VCALENDAR
BEGIN:VEVENT
DTSTART;VALUE=DATE:20120302
DTEND;VALUE=DATE:20120302
RRULE:FREQ=MONTHLY;INTERVAL=1;BYDAY=1FR,3FR,5FR
DESCRIPTION:
LOCATION:
SEQUENCE:0
STATUS:CONFIRMED
SUMMARY:Event on the first, third, and fifth Fridays.
TRANSP:TRANSPARENT
END:VEVENT
END:VCALENDAR
To import the ICS file (updated June 2018): Click the gear icon and select "Settings", then choose "Import & export" from the menu. (Here's a direct link as of June 2018.) Under "Import", select your ICS file and upload it to the desired calendar.
For this example, Google Calendar parses the RRULE correctly and even describes the recurrence as "Monthly on Friday of weeks 1, 3, 5 of the month":
Of course, you won't be able to tweak the custom recurrence rule from within Google Calendar, unless you want to replace the recurrence rule entirely with one of the standard options. Otherwise, you'll have to delete the event, tweak your ICS file, and reimport. You should still be able to make other changes to the event within Google Calendar, such as renaming, moving to another calendar, etc.
If this helps, you may wish to read more about the flexibility offered by custom recurrence rules in the iCalendar specification.
Best Answer
This isn't a built-in feature of Calendar, it doesn't assume people's birthdays are on the 1st - if a birthday is missing, it's not displayed. This seems like a 3rd party app added birthdate data and guessed.