@Jack Nye,Thanks for posting in Q&A.
From your description, I know you want to display a banner at the top of the login screen and have the time and date displayed there.
Based on my research, we can create a device configuration policy with Interactive Logon Message Text For Users Attempting To Log On settings configured, you can add some message in it, but as for adding date and time, there is no such methods mentioned.
Hope it will help.
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