DateTime conversion to Excel Date in f# - f#

Question

I read on stackoverflow that easiest way to convert a DateTime variable back to Excel date was simply to do:
let exceldate = int(DateTime)
Admittedly this was in c# and not f#. This is supposed to work as the decimals represent time and int part represents date. I tried this and f# comes back with the error:
The type 'DateTime' does not support a conversion to the type 'int'
So how do I convert back to excel date?
More specificly, I m trying to create a vector of month 1st for a period between start date and end date. Both vector output and start date and end date are floats, i.e. excel dates. Here my clumsy first attempt:
let monthlies (std:float) (edd:float) =
let stddt = System.DateTime.FromOADate std
let edddt = System.DateTime.FromOADate edd
let vecstart = new DateTime(stddt.Year, stddt.Month, 1)
let vecend = new DateTime(edddt.Year, edddt.Month, 1)
let nrmonths = 12 * (edddt.Year-stddt.Year) + edddt.Month - stddt.Month + 1
let scaler = 1.0 - (float(stddt.Day) - 1.0) / float(DateTime.DaysInMonth(stddt.Year , stddt.Month))
let dtsvec:float[] = Array.zeroCreate nrmonths
dtsvec.[0] <- float(vecstart)
for i=1 to (nrmonths-1) do
let temp = System.DateTime.FromOADate dtsvec.[i-1]
let temp2 = temp.AddMonths 1
dtsvec.[i] = float temp2
dtsvec
This doesnt work because of the conversion issue and is rather complicated and imperative.
How do I do the conversion? How can I do this more functionally? Thanks

Answer 1

Once you have the DateTime object, just call ToOADate, like so:
let today = System.DateTime.Now
let excelDate = today.ToOADate()
So your example would end up like so:
let monthlies (std:float) (edd:float) =
let stddt = System.DateTime.FromOADate std
let edddt = System.DateTime.FromOADate edd
let vecstart = new System.DateTime(stddt.Year, stddt.Month, 1)
let vecend = new System.DateTime(edddt.Year, edddt.Month, 1)
let nrmonths = 12 * (edddt.Year-stddt.Year) + edddt.Month - stddt.Month + 1
let scaler = 1.0 - (float(stddt.Day) - 1.0) / float(System.DateTime.DaysInMonth(stddt.Year , stddt.Month))
let dtsvec:float[] = Array.zeroCreate nrmonths
dtsvec.[0] <- vecstart.ToOADate()
for i=1 to (nrmonths-1) do
let temp = System.DateTime.FromOADate dtsvec.[i-1]
let temp2 = temp.AddMonths 1
dtsvec.[i] = temp2.ToOADate()
dtsvec
In regards to getting rid of the loop, maybe something like this?
type Vector(x: float, y : float) =
member this.x = x
member this.y = y
member this.xDate = System.DateTime.FromOADate(this.x)
member this.yDate = System.DateTime.FromOADate(this.y)
member this.differenceDuration = this.yDate - this.xDate
member this.difference = System.DateTime.Parse(this.differenceDuration.ToString()).ToOADate
type Program() =
let vector = new Vector(34.0,23.0)
let difference = vector.difference