Once you know this:
Date
by calling the constructor with milliseconds since Jan 1, 1970.valueOf()
a Date
is the number of milliseconds since Jan 1, 197060,000
milliseconds in a minute :-]...it isn't so hard.
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);