Possible to aggregate by month, day, etc?
It depends on your database.

For sqlite you might use the `strftime` function to truncate a timestamp. This would calculate a sum of some value for each month:

``` python
query = (Values
         .select(fn.Sum(Values.value).alias('average_value'))
         .group_by(fn.strftime('%Y-%m', Values.timestamp))
         .tuples())
```

For postgresql you would use `date_trunc`.

``` python
query = (Values
         .select(fn.Sum(Values.value).alias('average_value'))
         .group_by(fn.date_trunc('month', Values.timestamp))
         .tuples())
```
 OK, was looking for something that wasn't as dependent on the underlying DB, but I can use that for now.  Thanks.
 Yeah, it might be a good idea to create a custom `date_trunc` function for sqlite. As far as MySQL, I'm not really sure since I don't use MySQL myself.
 Fixed in b6b6c2e9ed7215e25494c8bf4b8a73118eedc724

What's the recommended way to do a group by month or day with a DateTimeField?  I know some of the underlying databases have date functions, and I know I could do it by formatting it as a string and truncating the hours, minutes, etc. but I thought there might be a more portable way to do it using Peewee.
