How do I subtract minutes from a date in javascript?

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);

You can also use get and set minutes to achieve it:

var endDate = somedate;

var startdate = new Date(endDate);

var durationInMinutes = 20;

startdate.setMinutes(endDate.getMinutes() - durationInMinutes);