Comment 3 for bug 1649086

The solution is to tell the owners of the respective repositories to fix their Release file(s). The Date (and Valid-Until) field MUST be in UTC (aka GMT, Z, +0000). Earlier apt versions accepted other timezones silently, but parsed it as UTC anyhow which could cause all kinds of fun. Now a /warning/ is generated – a user can work with the repository as before. As the content of that field is only of use for applications and apt isn't a fullblown calendar-application we decided to not implement all the craziness which is time.

All hints for creating repositories which apt (and other clients and servers) can work with: https://wiki.debian.org/RepositoryFormat

So, as this isn't a bug: Closing as invalid.