As promised, Sony is now sending out emails to PlayStation Plus users, offering a one-day membership extension to those affected by the PlayStation Network downtime at the beginning of the month.
In the email, Sony explains that you have until March 31, 2016 to redeem the code for your extra day:
Due to the recent PlayStation Network interruption we are giving you a 1-day extension to your PlayStation Plus membership. Please redeem the [voucher code] by March 31, 2016, 11:59pm PT.
Sign in to redeem your code.
The extension will be applied to the end of your current membership. This applies to all accounts that had an active PlayStation Plus membership on January 4, 2016.
Thanks for your patience during the service interruption.
PlayStation Now and video rentals will also receive extensions, though we don’t know for how long.
If you haven’t received your email yet, expect it to arrive within the next few days.