Add `toDateTime(jsDate, precision)` helper function

[AnyhowStep]

Would be nice to have these helper functions or something similar,

function toDateTime0 (jsDate : Date) : Date {
    return new Date(Math.floor(jsDate.getTime() / 1000) * 1000);
}

function toDateTime1 (jsDate : Date) : Date {
    return new Date(Math.floor(jsDate.getTime() / 100) * 100);
}

function toDateTime2 (jsDate : Date) : Date {
    return new Date(Math.floor(jsDate.getTime() / 10) * 10);
}

Playground