Clock and date settings are all correct.
But I've noticed that my son's computer has not managed to download the lastest Windows 10 updates, and that there are numerous 'family' notifications in the Action Center. Now, I read somewhere that sites can often be declared insecure on Firefox on the basis of microsoft account settings, so am now thinking that the problems lies here. But when I entered the accounts section on the computer, I was not able to delete or change individual profiles.