@Tobias ******
whether they add the 60 day extension on to the end of your current stamps expiration date OR start it on the day you apply is totally up to the immigration office.
If it's your first covid extension MOST offices are starting it on the date you apply because it's a different reason for your extension
GO to your immigration office and ask them, although it really won't matter, you are only needing the extension to get the funds seasoned so you can apply for the year so even if the 60 days starts on the date you apply seeing as your money is here already it's enough time