Calendar popup prints multiple dates in October when client is on Brazilian timezone

When your operating system has its timezone set to "Brazil" or "America/Sao_Paolo" or "GMT-3", there is a peculiarity in Javascript whereby the following happens:

Evaluating
javascript:document.innerHTML=new Date(2009,09,17)
produces the output

Sat Oct 17 2009 00:00:00 GMT-0300 (BRT)

and

evaluating
javascript:document.innerHTML=new Date(2009,09,18)
produces the output

Sat Oct 17 2009 23:00:00 GMT-0300 (BRT)

This is causing a bug in our calendar popup picker used for dates in JIRA, where for instance in 2009, the 17th of October is repeated twice, as shown in the screenshot. The problem repeats every October.

Similar problems could exist with other timezones and months. Brazil is different from Australia in that DST takes effect at Midnight.

The problem with the date picker only occurs when:

• The time component is set to Midnight (i.e. its a date-only picker, or a date-time picker with the time set to Midnight)
• There is already a value set for the component