thank you for your quick and comprehensive answer.

Of, course

    declare function local:dateTime-to-ms($point as xs:dateTime) as xs:decimal {
        ($point -xs:dateTime("1970-01-01T00:00:00")) div xs:dayTimeDuration('PT0.001S')
    };

usage

    local:dateTime-to-ms(xs:dateTime("1970-01-02T00:00:00"))

could mimic aspects of convert:dateTime-to-ms until a final release of 7.5 is there.

With best regards

Andreas

P. S.:

Since XQDT works under Eclipse 3.7 and offers parentheses matching
I'm afraid I'll not get the resources to sponsor the feature under BaseXGui ;-)