You have to include the entire timestamp, including the milliseconds and the offset from GMT (UTC).

Here is an example.

Note, I have hard-coded the offset as "+0", and simply subtracting "1" from the hour is a bit less than smart, but I'll leave you to sort those out

Code:
$=SetOption("Explicit","ON")
$=SetOption("WrapAtEOL","ON")
$=SetOption("ASCII","ON")
 
Dim $sEvtLog,$sEvtType,$sEvtTimestamp
Dim $oEvents,$oEvent
Dim $asNow
 
$sEvtLog = "System"
$sEvtType = "Information"
 
$asNow=Split(@TIME,":")
$sEvtTimestamp=CStr(Join(Split(@DATE,"/"),""))
	+CStr(Right("0"+(Cint($asNow[0])-1),2))
	+$asNow[1]
	+$asNow[2]
	+".000000"
	+"+0"		; OFFSET FROM GMT (in minutes)
 
$sEvtTimeStamp ?
 
$oEvents = GetObject("Winmgmts:\\@WKSTA").ExecQuery("
	Select * from Win32_NTLogEvent
	Where
		Type = '"+$sEvtType+"'
		AND Logfile = '"+$sEvtLog+"'
		AND TimeWritten >= '"+$sEvtTimestamp+"'
	")
 
For Each $oEvent In $oEvents
	"Orig EVT date time: " + $oEvent.TimeWritten ?
Next

Exit 0