CPAN Statistics API for mswin32 build failures

I wanted to look into CPAN module build failures on mswin32 for versions of
Perl >= 5.10

I found a link to the CPAN Testers REST API, but am not exactly sure how
the authors want others to use it, or where it is correct usage is
documented.

Example: What is the precise format for date/time?  I've tried a number of
alternatives, but all of them report:

{"errors":[{"message":"Does not match date-time
format.","path":"\/since"}],"status":400}

Can anyone here give a bit of guidance? Thanks.