There are a couple ways to use the importer to retain your Google Analytics data when migrating to Matomo. This section details possible workflows you can use.
Setup tracking to your Matomo and import into a site with an end date
This is the simplest approach in terms of setting up and monitoring. To do it, Start importing your old data and set the end date to the day you start tracking with Matomo (today’s date for example), use the tracking code from the newly created site and start tracking.
Make sure there’s no overlap in the import end day and the day you start tracking. So, for example, if you start tracking on November 12th, make sure the import end date is November 11th.
Then end your tracking with GA whenever you’re ready to. This solution is also explained in more details in the Migrating from Google Analytics FAQ.
Import into a site and let the import continue until you are ready to stop tracking into GA
Another option is to have an ongoing import into your Matomo while still tracking into GA. For this approach you would start an import without specifying an end date. This will cause the import to just keep importing every day.
When you are ready to switch to Matomo, you can manually cancel the import and start tracking into Matomo (make sure you are ready to start tracking before you cancel the import, since if it takes a couple days for you to setup, those days won’t have data in Matomo). Then you can stop tracking into GA.
This approach could be useful if you are evaluating Matomo but are not ready to invest time in setting up tracking to your Matomo.