There are a few things that might cause a date different than what you expected:
Be sure to launch Rocket League on all linked platforms. Follow all prompts and make sure you see the Main Menu on each account. Shared Inventory, including this title, is only enabled after completing this step.
Once finished opening each platform look for the expected date title on your main account:
- Launch Rocket League on your main account
- Select Profile
- Select the Tag icon to view your Titles
The EST. 20XX is set the first time you logged onto the Rocket League servers. If you bought Rocket League in 2015 but played offline until 2016, you would get the title EST. 2016.
Active Platform Subscription
An active platform subscription (PlayStation+, Xbox Live Gold, Nintendo Switch Online) was required to connect with Rocket League servers. The EST. 20XX Title dates to the first year with both an online connection and an active platform subscription.