Summary

When the user have the profile language in Japanese and he tries to add a date on the "Activity Stream Gadget" using the calendar function it looks like automatically add a non needed double byte value, on this case "月" that means "month" And is causing the error of the attached images, and the user is unable to select/save the specific date.
Even typing on the correct format is not possible to save it.

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The error if translated to english says：
入力した日付は無効です。推奨する書式は "2014/11月/14" です。
The date you entered is invalid. The recommended format is "2014/11 月 / 14".
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If the user changes the profile language to other language it works fine.
This was reproduced on the following versions of JIRA:
JIRA Server
6.2.7, 6.3.8, 6.3.10
JIRA Cloud
6.4-OD-09-008

Workaround

• Delete '月' character and hit 'Enter' key (see Workaround.jpg also)