Steps to Reproduce

1. Set the UI language to Japanese ("日本語") at https://id.atlassian.com/manage-profile/account-preferences
2. Access a backlog
3. Select End Date from the date picker
4. Click "Start(開始)"

Expected Results

The sprint will start

Actual Results

An error is showing saying an invalid format is entered.
Checking the network console, POST rest/greenhopper/1.0/sprint/<N>/start is returning 400 error

Error message:
 

'18/9/23 05:25 AM' は有効な日付ではありません。次の形式で日付を入力してください：dd/MMM/yy h:mm a

This also occurs when editing the sprint start / end dates after creating the sprint.

Workaround

We'll be bringing an enhanced board and backlog to Company-managed Projects in the next few months which will resolve this bug: 
Enhanced Jira board and backlog

When enhancements are rolled out and available for you, you’ll see a toggle in the view menu, which you can use to switch on and off the enhancements. Note that enhancements are applied per user — you’ll be switching them on for your board and backlog, and not anyone else’s

Follow the updates on our public Cloud Roadmap

*** 

This issue occurs only when Date/Time format on Look and Feel settings are not matched with the one on System Advanced Settings (jira.date.time.picker.java.format/jira.date.time.picker.javascript.format).

For that reason, as a workaround,  (1) change the date/time format in the Jira System Settings. See this community article for more information

Or, (2) reverting the Date/Time format on Look and Feel settings to its default value by clicking [Revert] button will resolve the issue.

If for some reason you are unable to apply the above changes, please use the following adhoc workarounds.

• (2-1) Change the UI to English at https://id.atlassian.com/manage-profile/account-preferences, or
• (2-2) After selecting the date, change the "AM" to "午前" or "PM" to "午後" manually.