Summary
When I choose a date from the date picker, Issue Editor raises an error saying that date format is invalid.
Answer
Set the Date Picker and Date Time Picker to match the format in Jira (System > Look and feel and System > General configuration > Advanced Settings).
Example
System > Look and feel
| Time Format | h:mm a |
| Day Format | EEEE h:mm a |
| Complete Date/Time Format | dd/MMM/yy h:mm a |
| Day/Month/Year Format | dd/MMM/yy |
System > General configuration > Advanced Settings
| jira.date.picker.java.format | d/MMM/yy |
| jira.date.picker.javascript.format | %e/%b/%y |
| jira.date.time.picker.java.format | dd/MMM/yy h:mm a |
| jira.date.time.picker.javascript.format | %e/%b/%y %I:%M %p |