Calculating Previous Month(s) From Current Date

Question

Greeting JavaScripting Gurus!
I would like to ask for some assistance on Date Calculation.
Here is my current scenario:
The user inputs a date mm/dd/yyyy ("DateField")
The powers that be over here are asking for the "DateField" to then calculate these items:
"DateField" minus(-) 12 months = mm/yyyy ("12MonthsAgoDateField")
"DateField" minus(-) 18months = mm/yyyy ("18MonthsAgoDateField")
"DateField" minus(-) 36 months = mm/yyyy ("36MonthsAgoDateField")
and so on...
I've read some of the Date Calculation posts, but could no seem to find anything that fit this scenario.
I freely confess my javascripting ability is very minor.
I would greatly appreciate any help in scripting this.
Thank you!

Answer 1

Unfortunately the numbers of days in a month or year are not the same for all months or years, so the get and set date methods might not work. Fortunately there are the getMonth() and setFullYear() methods for getting or setting the month or year for a given date.
For the "On Blur" action for the "DateField" you can use:
function GetField(cName) {
// get a field object with error catching;
var oField = this.getField(cName);
if(oField ==  null) app.alert("Error accessing field named: " + cName, 0, 0);
return oField;
} // end GetField function;
function AddMonths(oDate, nMonths) {
// add nMonths to oDate object;
oDate.setMonth(oDate.getMonth() + nMonths);
return oDate; // return adjusted date object;
} // end AddMonths;
function Scand(cFormat, cDate) {
var oDate = util.scand(cDateFormat, event.value);
if(oDate == null) app.alert("Error converting " + oDate.valueAsString + " with format: " + cDateFormat, 0, 0);
return oDate;
} // end Scand functon;
var cDateFormat = "mm/dd/yyyy"; // format for date strings;
// event value is the start date string;
// 12 months ago;
var o12MonthsAgo = GetField("12MothsAgoDateField")
var oDate = Scand(cDateFormat, event.value);
oDate = AddMonths(oDate, -12) // subtract 12 months;
o12MonthsAgo.value = util.printd(cDateFormat,oDate);
// 18 months ago;
var o18MonthsAgo = GetField("18MothsAgoDateField")
oDate = Scand(cDateFormat, event.value);
oDate = AddMonths(oDate, -18) // subtract 18 months;
o18MonthsAgo.value = util.printd(cDateFormat, oDate);
// 36 months ago;
var o36MonthsAgo = GetField("36MothsAgoDateField")
oDate = Scand(cDateFormat, event.value);
oDate = AddMonths(oDate, -36); // subtract 36 months;
o36MonthsAgo.value = util.printd(cDateFormat, oDate);
// and so on;
Since getting a field object, converting a date string to a date object and adjusting the date object using the getMonth and setMonth methods are repeated several times I have used functions so the repeated code could be reused.