How To Find Number of Days Between Dates Outside of Normal Range

   Function AstroDay(inyear, inmonth, inday)
   ' The AstroDay function returns the Astronomical Day for any given date.
   y = inyear + (inmonth - 2.85) / 12
   AstroDay=Int(Int(Int(367*y)-1.75*Int(y)+inday)-.75*Int(.01*y))+1721119
   ' NOTE: Basic's Int function returns the integer part of a number.
   End Function
				

Print AstroDay(12000, 3, 1) - AstroDay(12000, 2, 28)   'Prints 2
				

MORE INFORMATION

The AstroDay function defined in this article is a modified version of the Julian date formula used by astronomers.

Visual Basic's DateSerial function returns a Variant of VarType 7 (Date) containing a date that is stored internally as a double-precision number. This number represents a date from January 1, 100 through December 31, 9999, where January 1, 1900 is 2. Negative numbers represent dates prior to December 30, 1899.

Leap years are accurately handled by both Visual Basic's DateSerial function and the Astronomical Day function (AstroDay) defined in this article.

A leap year is defined as all years divisible by 4, except for years divisible by 100 that are not also divisible by 400. Years divisible by 400 are leap years. 2000 is a leap year. 1900 is not a leap year.

Step-by-Step Example

1. Start a new project in Visual Basic. Form1 is created by default.
2. Double-click the form to open the code window. Add the following code to the Form Load event:

      Sub Form_Load ()
   
         form1.Show  ' Must first Show form in Load event for Print to work.
         Print AstroDay(12000, 3, 1) - AstroDay(12000, 2, 28)  'Prints 2
         Print AstroDay(-12400, 3, 1) - AstroDay(-12400, 2, 28)  'Prints 2
         Print AstroDay(12000, 3, 1) - AstroDay(-12000, 2, 28) 'Prints 8765822
         Print AstroDay(1902, 2, 28) - AstroDay(1898, 3, 1)  'Prints 1459 days
         Print AstroWeekDay(AstroDay(1993, 12, 1))  ' Prints Wednesday
         Print AstroWeekDay(AstroDay(12000, 3, 2))  ' Prints Thursday
   
         ' You can also use Visual Basic's DateSerial function as follows to
         ' find the number of days between two dates:
         Print DateSerial(1902, 2, 28) - DateSerial(1898, 3, 1)
   
         ' Visual Basic's WeekDay function returns an integer betw 1 (Sunday)
         ' and 7 (Saturday), which represents the day of the week for a date
         ' argument:
         Print "Day of week = " & Weekday(DateSerial(1898, 3, 1))
   
      End Sub
   					

3. In the Object box on the Form1.Frm code window, select (general). Add the following functions:

      Function AstroDay(inyear, inmonth, inday)
      ' The AstroDay function returns the Astronomical Day for any given date.
      y = inyear + (inmonth - 2.85) / 12
      AstroDay=Int(Int(Int(367*y)-1.75*Int(y)+inday)-.75*Int(.01*y))+1721119
      ' NOTE: Basic's Int function returns the integer part of an number.
      End Function
   
      Function AstroWeekDay (aday)
      ' The AstroWeekDay function returns the day of the week, Sunday through
      ' Monday, for any given AstroDay. The aday parameter must be a day
      ' number returned by the AstroDay function.
      weekdayx = (aday - 3) Mod 7
      Select Case weekdayx
      Case 0
         AstroWeekDay = "Sunday"
      Case 1
         AstroWeekDay = "Monday"
      Case 2
         AstroWeekDay = "Tuesday"
      Case 3
         AstroWeekDay = "Wednesday"
      Case 4
         AstroWeekDay = "Thursday"
      Case 5
         AstroWeekDay = "Friday"
      Case 6
         AstroWeekDay = "Saturday"
      End Select
      End Function
   					

4. Start the program (or press the F5 key). Close the form to end the program.