calculating age in weeks and days

Help please 

if I need to calculate the gestational age in a certain date (any date)

how do I do please ?

I have 2 feilds 

the first "DG" date of conception

the second "age" the date at which i want to calcule the gestational age in weeks and days 

this my script (try67 helped me in this script) but this script is for the today date :

var dateFormat = "dd/mm/yyyy"; var startDateString = this.getField("DG").valueAsString; event.value = ""; if (startDateString!="") { var startDate = util.scand(dateFormat, startDateString); if (startDate!=null) { var now = new Date(); var dayInMs = 86400000; var diff = Math.round((now.getTime()-startDate.getTime())/dayInMs); var diffInWeeks = Math.floor(diff/7); var diffInDays = diff-(diffInWeeks*7); event.value = diffInWeeks + " SA " + diffInDays + " jours"; } }

please help me