JIRA's internal date picker component is used to select date-time in various places inside JIRA. WorkLogPRO Timesheet for JIRA add-on also uses this JIRA's build-in date-time selector. Inorder this component to work correctly 4 date-time related values should be consistent in JIRA's Advanced Settings page. If it is not, some functions which require date selection will not work. For example in WorkLogPRO, you will able to select a date using the picker but you will get "You must specify a date on which the work occurred" when you try to submit the work log. See following screenshot:
Adjusting Date-Time Picker Formats in JIRA
These settings are highlighted with green and yellow colors below.