Milestone time periods can vary from other report time frames. To set up  or change time frames for milestones:

Go to Organizer Dashboard:

Under the Settings tab, click on Milestones

Set up your milestones, then under Over what kind of time period, select one of the radio button options.

You can also set up a custom time frame. For example: if you want to set up hours for a school year that runs from September to June, you would select Every 1 Year starting on 9/1/2018. 

Milestones automatically reset after each specified timeframe, until changes are applied or turned off.

For more help with milestones, see this page.