Hello Cosmo_wanda,
Thank you for using the Apple Support Communities. I understand you're not seeing the date on your Lock screen.
Is this happening while your device is connected to the charger? If so, this would be normal behavior.
On the other hand, if you're not charging your device when you see this happening, there are a few things to consider.
Does your iPhone charge as expected? Also, are you using an Apple-certified charger? If you're running into charging issues outside of the battery percentage on the Lock screen, I recommend checking out the information here: If your iPhone, iPad, or iPod touch won't charge
Does it help if you Restart your iPhone, iPad, or iPod touch?
Also, which iOS version are you using under Settings > General > About? If you're not using iOS 12.1.4, I recommend updating: How to back up your iPhone, iPad, and iPod touch and Update your iPhone, iPad, or iPod touch.
I hope this helps!
Take care.