I put much more detailed thoughts on both immediate-term fixes and longer-term suggestions on GitHub where they were first requested:

But, within the scope of this conversation, it mostly boils down to adding one further qualification: "as long as it's a valid GNU date which matches the actual behavior of the system"

GNU date(1) interprets such "yyyy-mm-ddZ" constructions as 00:00 UTC, but in casual conversation, expiration dates are generally assumed to mean 24:00 by convention unless otherwise specified.  Depending on the actual behavior of your system, being more explicit at the expense of some verbosity may be a better longer-term/transitional choice.