This is a powershell script that will update all the times for alerts within a specified Sharepoint Site. You must change the url and time to match your needs.

UpdateSiteAlertTimes.ps1

cls
$error.Clear()

##################################################################################################
# Load SharePoint Snapin
##################################################################################################
$snap = Get-PSSnapin | Where-Object {$_.Name -eq 'Microsoft.SharePoint.Powershell'} 
if ($snap -eq $null) { 
  Write-Host "Loading Powershell Snapin..." -ForegroundColor Yellow
	Add-PSSnapin Microsoft.SharePoint.Powershell 
}

[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint") | Out-Null


##################################################################################################
# Global Variables
##################################################################################################
$Today = [string]::Format( "{0:yyyy-MM-dd}", [datetime]::Now.Date )

###Configuration###
$SiteCollectionUrl = "Enter Site URL Here"
$NewAlertTime = "2/26/2013 4:00:00 PM"

$RootWeb = get-spweb $SiteCollectionUrl

$TranscriptFileName = "$Today - List User Alerts.txt"



$ErrorActionPreference="SilentlyContinue"
Stop-Transcript | out-null
$ErrorActionPreference = "Continue"
Write-Host "Starting Transcript..." -ForegroundColor Yellow
Start-Transcript -path $TranscriptFileName

Write-Host "##########################################################################################" -ForegroundColor Yellow
Write-Host "# $Today - Running User Alerts Script" -ForegroundColor Yellow
Write-Host "# Script will update date/time that alerts will be distributed." -ForegroundColor Yellow
Write-Host "##########################################################################################" -ForegroundColor Yellow
Write-Host ""


##################################################################################################
# Loop through sites
##################################################################################################
foreach ($web in $RootWeb.Webs) 
{
	Write-Host “==================================” -ForegroundColor Green
	Write-Host ("Updating alerts for site: "+$web.Title)
	Write-Host “==================================”  -ForegroundColor Green
	
	foreach($alert in $web.Alerts)
	{

	Write-Host “Updating alert :” $alert.title -ForegroundColor Yellow	
	write-host “Subscribed User: ” $alert.user
	
	$alert.AlertTime = $NewAlertTime
	write-host “Alert updated to: ” $alert.AlertTime	
	
	}
	Write-Host “==================================” -ForegroundColor Green
	Write-Host ("Done updating alerts for:"+$web.Title)
	Write-Host “==================================”  -ForegroundColor Green
}


#end

Write-Host "Script Completed....`n`n" -ForegroundColor Yellow
Stop-Transcript