setting date manualy in the field with ISO format (2017-08-08) updates the datepicker correctly.
save
click edit button again
=> same error, date in datepicker is 8-08-20

datepicker in v1.3.10 is also using ISO format, but is correctly handled

mantis1310_bug_date_05.PNG (16,022 bytes)   

mantis1310_bug_date_05.PNG (16,022 bytes)   

Not sure if This should be a new bug or not, Date picker always switch to '١٢٣٤٥٦٧٨٩٠' numbers and this causes issue while saving.
Note: Operating System Used is "mac os", keyboard "Arabic-PC"
Steps to reproduce:
1-Change preferred language in account preference to Arabic
2-Go to manage project
3- Add new version
4-Edit version
5-Do any change & save

Expected: Data should be saved correctly
Actual: Gets "APPLICATION ERROR 0000021"

Screen Shot 2017-11-17 at 1.02.03 AM.png (32,075 bytes)   

Screen Shot 2017-11-17 at 1.02.03 AM.png (32,075 bytes)   

Screen Shot 2017-11-17 at 1.01.29 AM.png (28,110 bytes)   

Screen Shot 2017-11-17 at 1.01.29 AM.png (28,110 bytes)   

Screen Shot 2017-11-17 at 1.00.51 AM.png (26,895 bytes)   

Screen Shot 2017-11-17 at 1.00.51 AM.png (26,895 bytes)   

Please check if setting $g_datetime_picker_format fixes the issue.
Especially have a look at the note and the warning at https://www.mantisbt.org/docs/master/en-US/Admin_Guide/html-desktop/#admin.config.date

Setting the Property $g_datetime_picker_format fixes the issue for me.

Thanks @sanschm for confirming.
I would like also the reporter @lbayle2 to confirm before I close the issue.

I checked on MantisBT v2.12.0 (CentOS7, PHP7)
$g_datetime_picker_format='d-m-Y H:i';

The field 'Date Order' is now empty (previously a wrong date was displayed)
clicking on the date picker opens current month (april 2018)

So the expected date is not displayed.

am I setting $g_datetime_picker_format correctly ?

No time to check at the moment, but this should work

$g_datetime_picker_format = 'DD-MM-Y HH:mm';

validated, $g_datetime_picker_format fixes the problem.

it's a bit confusing that the date format is different from the other date variables, but pb is fixed so it's all good.

thank you all !