You signed up for My Time Dining when you booked right? (rather than one of the preset times) If so...

Go into the cruise planner, and then hover over dining, then there should be an option for My Time Dining. Click that. Then click "Add to Cart", Then choose your guests, then choose your day and time. and go through the process of checking out. It's free. You'll then get a confirmation email.

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Me (45), Wife (43), DD(16), DD(14), DS(11)
The information below is only for context to my comments. Not some sort of measuring tool.
RCCL: 2012 Freedom-7 night Western2014 Independence-8 night Eastern2014 Independence-6 night Western2015 Oasis-7 night Eastern2016 Navigator-8 night Eastern2018 Liberty-7 night Western2019 Navigator-9 night Southern
Celebrity: 2017 Celebrity Equinox-7 night Eastern