Sometimes you might want (or need) to change the default date-time format set in JIRA. Out of the box the format is set to d/MMM/yy, which translates to for instance 1/Jan/13.
Luckily, this setting can be easily edited through the administration interface under System » General Configuration » Advanced (scroll down the General Configuration page), as shown below:
In order to change the format for dates and date-times you need to look for four options:
In case your settings do not match you will get an error such as “Please enter the date in the format “d/MMM/yy” when trying to save dates using the date picker.
Possible combinations for these format specifications can be found on Atlassian’s Configuring date picker formats page.
Look and Feel
Optionally. you can edit the way dates and datetime are displayed to users through the Look and Feel section in the administration interface (System » User Interface » Look and Feel):