Birthdays and Work Anniversaries are set when it is entered by the other person in one of your one-on-ones. 

Once you set your colleague’s birthday and work anniversary in your one-on-one conversation, everyone else who has a one-on-one with that person will see the same dates reflected in that one-on-one conversation under “Important Dates”.

If the dates are changed in any of the one-on-ones that person is part of, the change will be reflected across all their one-on-one conversations. 

Users are currently unable to set or change their own birthdays or work anniversaries but this feature is coming soon!