Date & time keeps changing randomly, it means its settings gone wrong, you need to update the setting to set time manually.
To fix the date and time issue we refer to an article which help you to solve this issue, Click here.
Yes, your windows defender should be turned ON. If it keeps turning off then the Viruses & Malwares can come easily into your computer.
We don’t recommend to use ONLY windows defender security because it is not a good security than Norton and AVG security etc. for PCs. We’ll recommend you Norton Security, AVG Security, Mcafee etc.