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.
If you select "All", every snippet in your project will be included in your mash.
Tapping "Custom" will give you the ability to mash any period, no matter how long or short it is or if it covers several months or years.
First, you'll need to choose the starting date.
Then, select the ending date.
Tap "Continue" to create your custom mash.
That's it! Now you can preview the outcome.