When you tap on the mash button in a project, you'll see that this year's snippets are mashed by default, but this is not carved in stone. You can always select a different time frame to mash; it's up to you!
To do so, tap "Choose Dates" in the mashing screen.
Then, you'll be presented with several options of dates to mash.
You'll notice that Year > [current year] is selected by default.
In both the "Week" and "Month" options you can choose to mash either the current week/month or the past week/month.
Tap "Done" to continue.
Tapping "Customize" ("Custom" in the Android app) opens a menu in which you can mash all your snippets, a custom range, or arbitrary snippets.
To mash All the snippets in the project, tap on "Select (All)."
To mash a custom period, no matter how long or short it is or if it covers several months or years, press and drag your finger through the calendar.
To create an arbitrary mash, pick any snippets you want by tapping on their thumbnails.
Tap on the "Add to Mash" button to save your custom selection.
That's it! Now you can preview the outcome.