Hi, a bit more detail on the DayNumber option (which applies to the normal DateTimeGroupDescription class as well as the Queryable one). 
When selecting a datetime step to add then you get the following:
Year is the 4 digit year
Month is the Short Month name
Hour is the 24-Hour format hour number
Minute is the minute number
Second is the second number

However, Day is "<Short Month Name>-<Day Number>". Setting aside the fact that this format is incorrect in lots of cultures, this means that you cannot, for example, have the day number as the column and month as the row to give you a reasonably compact grid for a year. 
Instead every single day has to be listed in either the rows or columns. 
Or a similar scenario in a chart based on the PivotChartViewModel, displaying separate lines for each month with days on the x-axis.

My preference would be to make Day consistent with the rest, but given that would probably break some backwards compatibility, I suggest a DayNumber option too.