以下假设公历将在未来的五百八十四亿五千万年内继续有效。但是要做好失望的准备；随着我们的太阳开始膨胀，改变地球的轨道和一年的长度，日历可能会被废除，并且当地球在七十五亿年后坠入太阳时，很可能会采用其他措施。
顺便说一下，我甚至不尝试处理格里高利历之前的日期。我只返回日期在1582年10月15日之前发生的天数，需要能够表达这种返回值的原因是GetDateFromSerial函数具有asString参数。
Sub GetDateFromSerial(ByVal dateSerial As ULong, ByRef year As Long, ByRef month As Integer, ByRef dayOfMonth As Integer, ByRef secondsIntoDay As Integer, ByRef asString As String)
Const SecondsInOneDay As ULong = 86400 ' 24 hours * 60 minutes per hour * 60 seconds per minute
'Dim startOfGregorianCalendar As DateTime = New DateTime(1582, 10, 15)
'Dim startOfGregorianCalendarInSeconds As ULong = (startOfGregorianCalendar - New DateTime(1, 1, 1)).TotalSeconds
Const StartOfGregorianCalendarInSeconds As ULong = 49916304000
secondsIntoDay = dateSerial Mod SecondsInOneDay
If dateSerial < StartOfGregorianCalendarInSeconds Then
year = -1
month = -1
dayOfMonth = -1
Dim days As Integer = (StartOfGregorianCalendarInSeconds - dateSerial) \ SecondsInOneDay
asString = days & IIf(days = 1, " day", " days") & " before the adoption of the Gregorian calendar on October 15, 1582"
Else
'Dim maximumDateValueInSeconds As ULong = (DateTime.MaxValue - New DateTime(1, 1, 1)).TotalSeconds
Const MaximumDateValueInSeconds As ULong = 315537897600
If dateSerial <= MaximumDateValueInSeconds Then
Dim parsedDate As DateTime = DateTime.MinValue.AddSeconds(dateSerial)
year = parsedDate.Year
month = parsedDate.Month
dayOfMonth = parsedDate.Day
Else
' Move the date back into the range that DateTime can parse, by stripping away blocks of
' 400 years. Aim to put the date within the range of years 2001 to 2400.
Dim dateSerialInDays As ULong = dateSerial \ SecondsInOneDay
Const DaysInFourHundredYears As Integer = 365 * 400 + 97 ' Three multiple-of-4 years in each 400 are not leap years.
Dim fourHundredYearBlocks As Integer = dateSerialInDays \ DaysInFourHundredYears
Dim blocksToFactorInLater As Integer = fourHundredYearBlocks - 5
Dim translatedDateSerialInDays As ULong = dateSerialInDays - blocksToFactorInLater * CLng(DaysInFourHundredYears)
' Parse the date as normal now.
Dim parsedDate As DateTime = DateTime.MinValue.AddDays(translatedDateSerialInDays)
year = parsedDate.Year
month = parsedDate.Month
dayOfMonth = parsedDate.Day
' Factor back in the years we took out earlier.
year += blocksToFactorInLater * 400L
End If
asString = New DateTime(2000, month, dayOfMonth).ToString("dd MMM") & ", " & year
End If
End Sub
Function GetSerialFromDate(ByVal year As Long, ByVal month As Integer, ByVal dayOfMonth As Integer, ByVal secondsIntoDay As Integer) As ULong
Const SecondsInOneDay As Integer = 86400 ' 24 hours * 60 minutes per hour * 60 seconds per minute
If (year < 1582) Or _
((year = 1582) And (month < 10)) Or _
((year = 1582) And (month = 10) And (dayOfMonth < 15)) Then
Throw New Exception("The specified date value has no meaning because it falls before the point at which the Gregorian calendar was adopted.")
End If
' Use DateTime for what we can -- which is years prior to 9999 -- and then factor the remaining years
' in. We do this by translating the date back by blocks of 400 years (which are always the same length,
' even factoring in leap years), and then factoring them back in after the fact.
Dim fourHundredYearBlocks As Integer = year \ 400
Dim blocksToFactorInLater As Integer = fourHundredYearBlocks - 5
If blocksToFactorInLater < 0 Then blocksToFactorInLater = 0
year = year - blocksToFactorInLater * 400L
Dim dateValue As DateTime = New DateTime(year, month, dayOfMonth)
Dim translatedDateSerialInDays As ULong = (dateValue - New DateTime(1, 1, 1)).TotalDays
Const DaysInFourHundredYears As ULong = 365 * 400 + 97 ' Three multiple-of-4 years in each 400 are not leap years.
Dim dateSerialInDays As ULong = translatedDateSerialInDays + blocksToFactorInLater * DaysInFourHundredYears
Dim dateSerial As ULong = dateSerialInDays * SecondsInOneDay + secondsIntoDay
Return dateSerial
End Function
