我怎样才能把这个伪代码翻译成工作的 JS [不要担心结束日期来自哪里，除非它是一个有效的 JavaScript 日期]。

var myEndDateTime = somedate;  //somedate is a valid JS date  
var durationInMinutes = 100; //this can be any number of minutes from 1-7200 (5 days)

//this is the calculation I don't know how to do
var myStartDate = somedate - durationInMinutes;

alert("The event will start on " + myStartDate.toDateString() + " at " + myStartDate.toTimeString());