This is about https://phabricator.wikimedia.org/maniphest/query/advanced/ I guess.
I agree that something like this would be useful. Currently there is not even a hint about the date format expected. Users need to try blindly.
Phabricator has a date picker widget for forms, see https://secure.phabricator.com/uiexample/view/PhabricatorFormUIExample/. The Sprint app is using it in our instance.
A task upstream from someone willing to submit a patch has good chances of success imho.
Historically, the reason this control is free text is so you can type "7 days ago" to select a relative date, creating a filter which always shows the last week of activity.
We now have a mixed-mode control which supports relative dates but also provides a date picker for absolute dates, but it hasn't reached this UI yet.