Date Math in Javascript

gistfile1.js

 // calculate end day
 // note, end day should be the final second 
 // of the final day (23:59:59) and 1 day 
 // before today
 var end_time = parseInt(new Date().setUTCHours(23,59,59,59)/1000 - 86400, 10),
 
// calculate start day, x number of days before
// end day, note start day should be the start
// of the day (00:00:00)
start_time = parseInt(new Date((end_time - (days_ago * 86400)) * 1000).setUTCHours(0,0,0,0) / 1000, 10);