Are you using GraphLab Create, or the SFrame package?
If you are using GraphLab Create, you may want to convert your SFrame to a TimeSeries. Then you can check out slice for arbitrary date ranges or group for grouping your data by month. Then you can examine each month's data separately.
If you are using the open-source SFrame package, TimeSeries has not made it over there yet (but it will!). In the meantime, you can do this little hack:
my_sframe['timeafterepoch'] = mysframe['timestamp'].astype(int)
mysframe[(my_sframe['timeafterepoch'] >= startofmonth) & (my_sframe['timeafterepoch'] <= end of month)]