Date variables

Using a date data type allows you to store a date-time entity with a value from midnight on 1. January, AD 100 until midnight at the end of 31. December, 9999.

Convention is that date-time variable names begin with the prefix dtm, as in

dtmStartDate

dtmBirthday

dtmZeroHour

(This is a convention, it's optional.)

Assigning a literal date or time to a date-time type variable uses the hash symbol, #, rather than a quote character as the delimiter

dtmMillenium = #January 1, 2000#

dtmZeroHour = #February 17, 1802 23:39#

dtmNearlyMidnight = #March 1, 2005 11:59PM#

Note that American date conventions have been used in these examples and that 12-hour and 24-hour time formats are both allowed. The sysem locale and the system's date format determine how VBScript handles dates. If the time is not specified then it is set to all zeros, 00:00 at the start of the day specified in the date. (This has implications when performing arithmetic on dates.)

Variables having a string data type can be converted to a date-time type using CDate, as in

strStringDate = "January 1, 2010"
dtmDateType = CDate(strStringDate)

This is, of course, equivalent to

dtmDateType = #January 1, 2010#

A number of key date functions are discussed in the example of determining the number of days in a month.

How dates are stored

VBScript stores dates in exactly the same way that Access does. Both VBScript and Access utilise the same COM structure to store date-time data. This is known as a vtDate Variant.

A VtDate is represented by a double-precision floating point number.

The integer portion (to the left of the decimal point) represents the day number - A series of day numbers beginning on 30. December 1899, with day +1 being 31. December 1899, as here:

dateSerial(Year(CDate(1)), _
   Month(CDate(1)), _
   Day(CDate(1))) = 31/12/1899

(Notice that when all is zero you can get some odd results, like this

dateSerial(Year(CDate(0)), _
   Month(CDate(0)), _
   Day(CDate(0))) = 00:00:00

which is a good case for using formatted dates in output)

The mantissa portion of the double precision floating point number (to the right of the decimal point) is the representation of the time during the day, some key values being

With 1440 minutes in a day, a minute is 1/1440, or 6.94444444444444E-04 , as the system would have it.

Back to the main article

Arithmetic on dates

VBScript lets you perform basic arithmetic operations on dates - Adding and subtracting an integer to and from a date variable moves the date into the future and back into the past by that number of days.

dtmDayTen = #March 10, 2005#
dtmFirstDay = dtmDayTen -9
dtmLastDay = dtmDayTen + 21

Then:

response.write "March begins on " & _
   dtmFirstDay & _
   " and ends on " & _
   dtmLastDay

result in the output

March begins on March 1, 2005 and ends on March 31, 2005

Adding and subtracting fractions of one works in a similar way, manipulating the time relative to the date-time value. Bearing in mind there are 1440 minutes in a day, using dtmValue + (60 / 1440) advances the variable an hour, for instance

dtmMidday = #May 2, 2004 12:00#
dtmTeaTime = dtmMidday + (360 / 1440)

where (360 / 1440) is 360 minutes, or six hours: Teatime is at six o'clock in the evening.

Since the way date-time values are manipulated is determined by the way the server it set up you may find that you get different results using the same ASP pages on different servers.

I've found that it is often more comfortable to work with potions of dates as strings, bringing them together when necessary and breaking them apart to create output elements. I prefer to use the format

day <dot> month year

which can reliably be rendered from

strDate = "22 September 1986"

using

response.write Day(strDate) & ". " & _
   MonthName(Month(strDate)) & " " & _
   Year(strDate)

output being

22. September 1986

in this instance.

If you would rather use the native date functionality then you will need to determine what settings are in place.

This can be done by reference to constants set by the VBScript runtime environment. So, for instance, to return a day of the week (the number of the day, starting with day one) you can use

Weekday(dtmDate)

If the date is the first day of the week it will return one, the next day 2 and so on. To determine which day of the week we are dealing with (Monday, Tuesday, Wednesday) we need to know on which day the server's week begins. We can use the constants vbSunday, vbMonday and so on (see the example) to make a comparison with the returned value or vbXxxxx to determine the weekday settings. Months are more straightforward, with January always being the first!