You could use the Approximate Count Distinct function to create a
calculated "Days" metric. It's only going to return a count for days
that have a value, but it sounds like that's what you want. Here's an
Analytics Demystified article explaining how to create a 'Daily Average
Visits' metric. You'd use the same concept, just take out the Visits
part. Should look like this: This is going to be Approximate, meaning
that sometimes it will be off by a small percentage, but hopefully it
will work for ...