Where are you talking about? On Garmin Connect, the Reports for total distance you have 7 days, 4 week, 6 months and 1 year. If you are looking at 4 weeks, then they go back from today's date 4 weeks. If you wait till the last week of this month, your 4 weeks past will be entirely within this month.

I think part of it is based on whether you set your week start day to be a Sunday or Monday. But I might be getting confused with RWGPS or other site.