NaN appears in date fields

In Jahia 5 we sometimes get a "NaN" message in the date field of the UpdateContainer engine dialogs.
It happens for instance for the field newsCl.date, which is filled automatically in a eventlistener. But it also happens for fields, which are not filled automatically, but set via the date picker. When opening the UpdateContainer-engine the value in the field suddenly reads "NaN".

We saw that on several occasions, but have not been able to reproduce it, so we do not know yet, when and why this happens.