Converting yearly data in monthly data

[RobertPrice]

I would like to convert yearly data into monthly data so that the monthly data is an interpolation of the yearly data. I would like the annual data to represent December of the relevant year and then for the following month (Jan) to be Dec plus 1/12th of the difference between the following Dec and the previous Dec.
i.e. Dec 1946: 48227
Dec 1947: 50494

Jan 1947: =(Dec1946+((Dec1947-Dec1946)/12))=(47227+((50494-48227)/12))=48415.92

I have attached an exampled spreadsheet where the original yearly data is in column B. The attempted formula is in column E. I was wondering whether somebody has written a macro for a similar problem which I could please apply to my problem.

Thanks for the help.

[Cutter]

See if this is something that will work for you.
Dates in column A have been converted to actual dates and dates in column D have been changed to end-of-month dates.

Formula in F2 and down:

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