Date Calculation - Start Date Plus Number of Months

Hello,

I have a form setup for a loan calculation. I have a begin date and end date. The end date is calculated based off the begin date plus a number of months.

So basically:

Number of Payments: 12
First Payment Date: May 10, 2010
Final Payment Date: April 10, 2010

I can't get the "FinalPaymentDate" to calculate unless I use a date in "FirstPaymentDate" that is past the ninth of a month. For example, if I chose my start date as May 5, 2010 the calculation will fail. If I use May 10, 2010 everything works just fine.

I also had to make a little tweak concerning the year. Whenever the calculation would calculate the month of December it would jump to the next year. For example, I would have 09/01/2010, 10/01/2010, 11/01/2010, 12/01/2011, 01/01/2011.

I'm not sure how to post an example of my pdf but if I figure that out I'll upload it.

Also, I take no credit for writing the code below. I took an example gkaiseril posted a few years ago and made minor tweaks to suite my needs. He gets and deserves the credit for writing the code.

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if(HasValue(FirstPaymentDate)) then
// get date and parts of date
var fStart = Date2Num(FirstPaymentDate.formattedValue, "MMM DD, YYYY")
// get month as number
var fMonth = Num2Date(fStart, "MM")
// get date as number
var fDate = Num2Date(fStart, "DD")
// get 4 digit year
var fYear = Num2Date(fStart, "YYYY")
// advance months
fMonth = fMonth + (NumPayments - 1)
// adjust year for added months
fYear = fYear + Floor(fMonth / 12)
// adjust month between 1 and 12
// get remainder of months divided by 12 months per year
fMonth = Mod(fMonth, 12)
// make a remainder of 0 a value of 12
if (fMonth == 0)then
fMonth = fMonth + 12
endif
// fix the issue of December going to next year
if (fMonth == 12)then
fYear = fYear - 1
endif
// format month to 2 numbers
fMonth = Format("99", fMonth)
// create Iso Date string
var sNewDate = Concat(fYear, "-", fMonth, "-", fDate)
// display result
Num2Date(IsoDate2Num(sNewDate), "MMM DD, YYYY")
else
null
endif