Fix createdAt date range filter

- hiddenName has been disabled in order to fix the missing date range
  values when using the material theme
- data format has been changed to 'Y-m-d' in order to comply with the
  browser date input default format
- tests: date() and strtotime have been replaced with DateTime-related
  objects

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>