Child Items do not respect "First day of the week" user setting

Summary

Child item dates (start, due to) do not respect the users "First day of the week" setting.

Steps to reproduce

Set "First day of the week" to monday, check the date-picker for child items

What is the current bug behavior?

First day of the week is shown as Sunday

What is the expected correct behavior?

First day of the week should be monday (/respect users "First day of the week" setting)

Relevant logs and/or screenshots