Adding date and time to Apple Watch face

Which watch face are you using?

Depending on the complications available, you can add it to a part of your watch face, check the related part of the Apple Watch User Guide:

Apple Watch faces and their features – Apple Support

For example the Modular Watch face:

What items do you have currently?

You need to use a watch face that has multiple complications (that's Apple speak for apps/features). I use the watch face Infograph which lets me add 8 complications to the face: date/calendar, messages, emails, pulse rate, blood oxygen, weather, countdown timers and stopwatch.

Open the Watch app on your iPhone and scroll thru the various faces. Pick the one that can add those complications you'd like to have:

You have a lot of flexibilities in designing your watch face. They are easily changed.