Date constraints should include call on date

[lognaturel]

Currently date constraints have XML output that looks like
"(. >= '2018-01-01') and (. <= '2018-03-31')"

The XML output should instead be something like
constraint="(. >= date('2018-01-01')) and (. <= date('2018-03-31'))"

Reported at https://forum.opendatakit.org/t/problem-with-constraint-on-date-widget/12029/

[2math]

It is strange, that when I change the XForm to constraint="(. >= date('2018-03-01’)) and (. <= date('2018-03-30’))” , then there is a parse error

System.err: org.xmlpull.v1.XmlPullParserException: Attr.value missing f. jr:itext (position:START_TAG <bind nodeset='/data/select_date' type='date' constraint='(. >= date('2018-03-01’)) and (. <= date('2018-03-30’))” jr:constraintMsg=' jr:itext='jr:itext('/data/group_items/input_number:constraintMsg')'>@149:154 in java.io.InputStreamReader@419a26e)

[2math]

Currently woks only with : constraint="(. >= 17590) and (. <= 17620)"
As double number which is "The number of days (as a double precision floating point) since the Epoch"