I am trying to count unique users on a monthly basis that were not present in the previous month. So if a user has a record for January and then another one for February, then I would only count January for that user.

```
user_id    time
a1         1/2/17
a1         2/10/17
a2         2/18/17
a4         2/5/17
a5         3/25/17

```

My results should look like this

```
Month   User Count
January     1
February    2
March       1

```