Question

How can I translate this pseudo code into working JS [don’t worry about where the end date comes from except that it’s a valid JavaScript date].

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()); 

Answer 1

Once you know this:

• You can create a Date by calling the constructor with milliseconds since Jan 1, 1970.
• The valueOf() a Date is the number of milliseconds since Jan 1, 1970
• There are 60,000 milliseconds in a minute :-]

In the code below, a new Date is created by subtracting the appropriate number of milliseconds from myEndDateTime:

var MS_PER_MINUTE = 60000; var myStartDate = new Date(myEndDateTime - durationInMinutes * MS_PER_MINUTE);