如何计算excel中一年中的某一天的日期？

Question

I've found plenty of functions to determine what day of the year a particular date is :

Function Yearday(dat As Date) As Integer
    'Purpose: does the same as day for month
    'Use to access an array where the index corresponds to days

    Yearday = DateDiff("d", CDate("1/1/" & Year(dat)), dat) + 1
End Function

But I haven't found any functions to determine the date, based on the day of a particular year.

The function signature would look like this:

Function getDateBasedOnYearAndDayOfYear(year As Integer, dayOfYear As Integer) AS Date

Answer 1

Maybe something like this,

Function getDateBasedOnYearAndDayOfYear(year As Integer, dayOfYear As Integer) AS Date
    getDateBasedOnYearAndDayOfYear = dateserial(year, 1, 0) + dayofyear
end function

Answer 2

Just create a new date with January first of the passed year, then add the number of days. This is really easy in VBA, because Dates are just doubles with the integer portion representing the number of days since 12/31/1899.

Start with this:

Function getDateBasedOnYearAndDayOfYear(year As Integer, dayOfYear As Integer) As Date

    Dim result As Date

    result = DateSerial(year, 1, 1)
    getDateBasedOnYearAndDayOfYear = result + dayOfYear - 1

End Function

I'd probably add bounds checking and so forth, ie the -634th day of the year doesn't make much sense.

Edit:

Just tested against DateSerial - you don't even need to do the math:

Function getDateBasedOnYearAndDayOfYear(year As Integer, dayOfYear As Integer) As Date

    getDateBasedOnYearAndDayOfYear = DateSerial(year, 1, dayOfYear)

End Function