This is a very weird problem, where i have a datetime field on a template, that returns null in some computers. I've tried in a same computer, with web browsers that never logged in in admin (just to make sure it wasn't about admin permissions over the field). I can only think it's a cookies/session issue, but still don't understand how this can happen on a field that comes from the $page object and not stored in cookies or session.
I'm using ProcessWire 2.7.2
It seems to be related to locale, I realised it was actually coming from just one of the languages of the website instead of randomly happening. I fixed it simply by filling some translations but feels like a bug from repeater fields in multilanguage.