If you login to an instance which is in a secure environmentand default language of the instance in [language code].[country code] format then it will corrupt the date picker (calendar field). This issue is not reproducible if you login to the instance in a normal environment.
It is recommended to use two character values with only [language code] to set the default language of the instance.
Ex: If default language is set as "en.GB" then, change to "en".
1. Below screenshot shows the corrupted 'date picker' in 'Change Request' record/form
2. Below screenshot shows default system localization properties with format [language code].[country code] that could result in a corrupted date picker
3. Modify the default language to format [language code] and save the property