Test plan:

1. Edit Date.format in js/common.js. It must be set accordingly to date_format in common/configuration.ini, otherwise dates won't be stored properly.
   Example:

Date.format = 'dd-mm-yyyy';
date_format = '%d-%m-%Y'

Or

Date.format = 'YYYY/mm/dd';
date_format = '%Y/%m/%d'

2. Check that the date format provided by the Datepickers in all the modules are correct.

3. Check that dates are stored properly.

Works as intended! Very nice!

The only (tiny) related issue I found is that in when editing Cost history there is a date format already filled in the empty row (in light gray), I suppose as a guide on how to write the date format. This date format isn't affected by the changes in this PR, nor PR 518

You're right. The placeholder was hardcoded. Fixed.