Check to see if a date/time is during Daylight Savings Time, or Standard Time

Category:

ASP, HTML and XML

Type:

Snippets

Difficulty:

Intermediate

Author:

Anonymous

Version Compatibility:

More information:

I needed a way using ASP to check a datetime value to see if it is during Daylight Savings or Standard time so that I could correctly show the timezone (standard or daylight), or accurately calculate the GMT value for the datetime. I wrote this function to help me accomplish this. It is written in vbscript for ASP, but could easily be converted to VB, VB.Net

Instructions: Copy the declarations and code below and paste directly into your VB project.

Declarations:
' (None)

Code:

Private Function CheckDayLightSavings(dtDateTime)
	'Author: Olin Hamilton
	'
	'Email:  olin@oct.net
	'
	'Pupose: This function returns true if a passed datetime value is in Daylight Savings time,
	'		 Returns false if the datetime value is in Standard Time
	'
	'Usage:  Use a date, or datetime formatted string as the parameter
	'		 If the parameter is not a valid date, the function will return False
	'		 If no Time part is passed with the string, it will assume the time is after
	'		 2 AM, so would return true on the first sunday of April, and False on the last Sunday in October.
	'		 Since a null time part for a datetime value defaults to exactly "00:00:00", if a date is passed with 
	'		 a time value equal to "00:00:00" the function will assume that the time value was not passed
	'		 and that the time is after 2 Am.  This may cause unexpected results if you are passing a valid datetime
	'		 value with a time value of exactly midnight.  Unfortunately, I could not think of any other way to handle
	'		 no time values, and figured it was best to consider it past 2AM.  
	'		 However, you can control this limitation by checking the time value for the date before passing it
	'		 to this function, and if it has a time value of "00:00:00", increasing it by one second will cause
	'		 the function to accurately return false if the date is the start day for DST,
	'		 or return True if it's the end day for DST.
	'		 If there are any questions, suggestions, comments, or modifications made, 
	'        please send me an email letting me know.
	
	Dim retVal, x, sTempDate
	'if the date time has the milliseconds, clean them off
	If InStr(1,CStr(dtDateTime),".") <> 0 Then
		dtDateTime = Left(dtDateTime, Len(dtDateTime) - 4)
	End If
	
	'If the passed string is a valid date, let's begin checking, otherwise
	'just return False.
	If IsDate(dtDateTime) Then
		'We know what to do with any dates within these months
		If Month(dtDateTime) <> 10 And Month(dtDateTime) <> 4 Then
			Select Case Month(dtDateTime)
				'Jan
				Case 1
					retVal = False
				'Feb
				Case 2
					retVal = False
				'Mar
				Case 3
					RetVal = False
				'May
				Case 5 
					retVal = True
				'Jun
				Case 6
					retVal = True
				'Jul
				Case 7
					retVal = True
				'Aug
				Case 8
					retVal = True
				'Sep
				Case 9
					retVal = True
				'Nov
				Case 11
					retVal = False
				'Dec
				Case 12
					retVal = False
			End Select	
		Else
			'If the month is April, let's check to see if the date is before or after
			'2 AM on the first Sunday of the month
			If Month(dtDateTime) = 4 Then
				If Day(dtDateTime) < 8 Then
					For x = 1 To Day(dtDateTime)
						sTempDate = CStr(Month(dtDateTime)) & "/" & x & "/" & CStr(Year(dtDateTime))
						If Weekday(sTempDate) = 1 Then
							If Day(sTempDate) < Day(dtDateTime) Then
								'First sunday in April has already passed, so we are now in DST
								retVal = True
								Exit For
							Else
								'It's the first Sunday in April!
								'Let's see if it's past 2 AM. If there is no time part in dtDateTime (time part = "00:00:00"), 
								'we are going to assume it's past 2 AM
								If (Hour(dtDateTime) >= 2) Or (Hour(dtDateTime) = 0 And Minute(dtDateTime) = 0 And Second(dtDateTime) = 0) Then
									retVal = True
									Exit For
								Else
									retVal = False
								End If
							End If
						Else 
							retVal = False
						End If 
					Next
				Else
					'we know what to do if the day is equal to or greater than the 8th
					retval = True
				End If
			'If the month is October, let's check to see if date is before or after
			'2 AM on the last Sunday of the month
			ElseIf Month(dtDateTime) = 10 Then
				'We know what to do if the day is less than then 25th
				If Day(dtDateTime) < 25 Then
					retval = True
				Else
					For x = 25 To Day(dtDateTime)
						sTempDate = CStr(Month(dtDateTime)) & "/" & x ...