Working with daylight saving time. (examples in VBscript)

Suppose you have a date and time and want to know if it is within the BST or the GMT period.

The first thing to do is calculate when BST and GMT start for the year of your date.

The following two functions return the start date and time for BST and GMT respectively...

Public Function BSTstarts(ByVal TheYear As Integer) As String
    BSTstarts = "March " _
    & 31 - (Weekday("#March 31, " & TheYear & "#", 2) Mod 7) _
    & ", " & TheYear & " 01:00:00"
End Function

Public Function GMTstarts(ByVal TheYear As Integer) As String
    GMTstarts = "October " _
    & 31 - (Weekday("#October 31, " & TheYear & "#", 2) Mod 7) _
    & ", " & TheYear & " 01:00:00"
End Function

...each function uses the year to calculate the respective start date and time.

The functions make use of the fact that both BST and GMT start at 01:00 hours GMT on the last Sunday of a particular month.

GMTstarts(2007) will return the start date and time of GMT in the year 2007.

Now that you have a means of determining when BST and GMT start for a given year you can now calculate which period your date and time is within.

To do this you can call a function that returns TRUE or FALSE depending on the result...

Public Function IsGMT(ByVal TheDateAndTime As String) As Boolean
    IsGMT = True
    If DateDiff("s", TheDateAndTime, _
    BSTstarts(Year(TheDateAndTime))) < 1 Then IsGMT = False
    If DateDiff("s", TheDateAndTime, _
    GMTstarts(Year(TheDateAndTime))) < 1 Then IsGMT = True
End Function

...this function uses the preceding two functions to calculate whether the start dates of BST and GMT have passed.

Each line of the function calculates the difference, in seconds, between your date and time and the start date and time of BST and GMT as returned by the functions BSTstarts and GMTstarts.

If the start date of BST has passed the result of the first calculation will be zero or negative.

The check for BST start is made first as this date occurs first in any year.
Now a check is made to see if the GMT start date has passed which will overide the previous result if necessary.

The returned value is 'True' if the date and time passed is within the GMT period, otherwise the result is 'False'.

At this point you have a means to determine which period a given date is within...

If IsGMT("12 October 2007 15:30:00") Then
    'It is within GMT
Else
    'It is within BST
End If

...which will return 'False' indicating that "12 October 2007 15:30:00" is within the BST period.

Next we will look at using this information to find the date of the next change and whether it will be from BST to GMT or from GMT to BST.

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