There are 4 settings which is related with “Date Time Picker” component used by “Issue Reminders” app. You can change format used but JavaScript and Java formats should be set to match each other. If you set different formats for Javascript and Java, wrong date will be displayed or reminders could be set for wrong dates.
Navigate to Administration->System→Advanced Settings. You will see 4 settings 2 for Javascript and 2 for Java.
jira.date.picker.java.format should match jira.date.picker.javascript.format
jira.date.time.picker.java.format should match jira.date.time.picker.javascript.format
These settings are highlighted with green and yellow colors below.
You can find several examples of consistent Javascript and Java date-time picker formats in this page.
