Events+ dates reverting to 1970-01-01

Hi There,
When creating events the dates stubbornly revert to 1970-01-01. This doesn't always happen. I don't see a pattern for when it happens or doesn't. It's a multi-site installation. There are multiple admins who create events. I found a couple of old posts in the forums about this, but nothing in there seems relevant. This is a much used, mission critical part of the site, so I need to address this quickly. I have activated support access; please be aware that this is a production site.
Thanks!
Richard