Which timezone decides what counts as today?

Your store's own timezone, taken from Shopify. Every day boundary in the product uses it: an order placed at 23:30 belongs to that day for you even if a system reporting in UTC would file it tomorrow.

It matters most at month boundaries, where a handful of late-evening orders can otherwise appear in different months in different reports. Keeping one timezone everywhere is also what makes a daily statement comparable day to day, and it is why the setting is read-only rather than something to be chosen twice.