How exactly is DAU / MAU calculated?

I'm reading a lot about DAU/MAU being a good metric for user engagement. But what is a DAU and what is a MAU?

For Daily Active Users: Is a user not a Daily Active User if they missed one day in the past 30 days?

For Monthly Active Users: Is a user a Monthly Active User if they were present once in the past 30 days?

Should I even be measuring by the past 30 days or should I go by calendar months?

'There are three kinds of lies: lies, damned lies, and statistics.' There are no hard and fast rules here - It means whatever the implementator decides is a valid action.

Example: Facebook defines Monthly Active Users as the number of unique users per the past 30 days.

Which then brings one to another question - what defines a unique user? In facebook land, these are the actions that are tracked / define it:

• mock ajax request
• fbml link click
• canvas page view
• click-to-play swf
• message attachment
• fbjs ajax
• accepting a request
• viewing a profile tab
• publishing something via a publisher As you can see, these actions are meaningless outside of facebook. To define your own MAU, you need to instrument actions that best describe activities that represent "active" use.

I used to do this by taking a user's span (say 6 months), seeing how many days he visited and averaging.

I have made a detailed write up after finding no complete article on it, about how to acheieve this in Google Analytics: http://www.notingon.com/dau-mau-measurements-in-google-analytics/