Setting end_date to a value smaller than start_date results in -xx days duration.

[mzimm]

If you set the end date of a ticket to a date before the ticket start date, it will accept the input and shows a negative duration.

There should be some basic input check.

[Ryan J Ollos]

That's out of scope for this plugin, but you could implement it using ​ITicketManipulator or in JavaScript. Please ask on the ​trac:MailingList if you have additional questions about that.