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Calculate datediff using SystemTime

Posted on 2007-11-27
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Last Modified: 2010-04-21
I need to calculate the difference in times down to the milliSecond.
DateDiff won't accept SystemTime so I wanted a native function to calc datediff down to the ms.

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Question by:sidwelle
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8 Comments
 
LVL 81

Expert Comment

by:zorvek (Kevin Jones)
ID: 20360266
If you are looking for the delta time between two points in your code use the VB Timer function. it returns a Single representing the number of seconds elapsed since midnight. It is accurate down to about 1/100 of a second.

Kevin
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LVL 55

Expert Comment

by:Jaime Olivares
ID: 20360316
if you just want the difference in miliseconds, you can use the WinAPI GetTickCount()
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LVL 55

Expert Comment

by:Jaime Olivares
ID: 20360345
Declare Function GetTickCount Lib "kernel32" () As Long

Dim t0 as Long
Dim t1 as Long
Dim diff as Long

    t0 = GetTickCount()
    ' Do some task here
    t1 = GetTickCount()

diff = t1 - t0   ' difference in miliseconds
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LVL 81

Expert Comment

by:zorvek (Kevin Jones)
ID: 20360383
The Windows SDK function GetTickCount has the same accuracy as the VB Timer function. I believe the Timer function uses GetTickCount.

Kevin
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LVL 81

Expert Comment

by:zorvek (Kevin Jones)
ID: 20360415
Here is a short article on getting better timing on Windows using VB and VB.net:

http://gpwiki.org/index.php/VB:Timers

Also note the following:

Windows NT 3.5 and later The system timer runs at approximately 10ms.
Windows NT 3.1 The system timer runs at approximately 16ms.
Windows 95 and later The system timer runs at approximately 55ms.

Kevin
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LVL 38

Expert Comment

by:PaulHews
ID: 20360450
Find difference in milliseconds between two systemtime variables:


Private Function GetDiffMilliseconds(Second As SYSTEMTIME, First As SYSTEMTIME) As Long
    'This function assumes the second date is the greater in'
    'order to return positive number differences.'
    'Returns number of milliseconds difference.
    
    Dim Ret As Long
    Dim dtFirst As Date, dtSecond As Date
    
    dtFirst = DateSerial(First.wYear, First.wMonth, First.wDay) + TimeSerial(First.wHour, First.wMinute, First.wSecond)
    dtSecond = DateSerial(First.wYear, First.wMonth, First.wDay) + TimeSerial(First.wHour, First.wMinute, First.wSecond)
 
    Ret = Second.wMilliseconds - First.wMilliseconds
    Ret = DateDiff("s", dtFirst, dtSecond) * 1000 + Ret
 
 
    GetDiffMilliseconds = Ret
End Function

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LVL 38

Accepted Solution

by:
PaulHews earned 250 total points
ID: 20360560
Woops, a couple of fixes.... Fix a typo and an overflow problem.
Private Function GetDiffMilliseconds(Second As SYSTEMTIME, First As SYSTEMTIME) As Double
    'This function assumes the second date is the greater in'
    'order to return positive number differences.'
    'Returns number of milliseconds difference.'
    
    Dim Ret As Double
    Dim dtFirst As Date, dtSecond As Date
    
    dtFirst = DateSerial(First.wYear, First.wMonth, First.wDay) + TimeSerial(First.wHour, First.wMinute, First.wSecond)
    dtSecond = DateSerial(Second.wYear, Second.wMonth, Second.wDay) + TimeSerial(Second.wHour, Second.wMinute, Second.wSecond)
 
    Ret = Second.wMilliseconds - First.wMilliseconds
    Ret = DateDiff("s", dtFirst, dtSecond) * CDbl(1000) + Ret
 
 
    GetDiffMilliseconds = Ret
End Function

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Author Closing Comment

by:sidwelle
ID: 31427345
Thanks Paul,
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