Time Zone Settings for O365 Groups SharePoint-Site

You can use the below script for this.

To Get the time zone index values 

https://msdn.microsoft.com/en-us/library/ms912391(v=winembedded.11).aspx

 To get Locale ID

https://msdn.microsoft.com/en-us/library/ms912047(v=winembedded.10).aspx

$cred= Get-Credential
$TimezoneValue= "(UTC+05:30) Chennai, Kolkata, Mumbai, New Delhi"
$localeid = 5129
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell -Credential $cred -Authentication Basic -AllowRedirection 
Import-PSSession $Session

#Add references to SharePoint client assemblies
Add-Type -Path ([System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client").location)
Add-Type -Path ([System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client.runtime").location)

$Groups =Get-UnifiedGroup |Where-Object {$_.SharePointSiteUrl -ne $null}|select SharePointSiteUrl

foreach($Group in $Groups.SharePointSiteUrl)
{
#Authenticate to Site
$Username =$cred.UserName.ToString()
$Password = $cred.GetNetworkCredential().Password 
$SecurePassword = $Password | ConvertTo-SecureString -AsPlainText -Force
$Site = $Group
$Context = New-Object Microsoft.SharePoint.Client.ClientContext($Site)
$Creds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Username,$SecurePassword)
$Context.Credentials = $Creds


$TimeZones = $Context.Web.RegionalSettings.TimeZones
$Context.Load($TimeZones)
$Context.ExecuteQuery()

#Changing  the timezone
$RegionalSettings = $Context.Web.RegionalSettings
$Context.Load($RegionalSettings)
$Context.ExecuteQuery()
$TimeZone = $TimeZones | Where {$_.Description -eq $TimezoneValue}
$RegionalSettings.TimeZone = $TimeZone
$RegionalSettings.Localeid = $localeid
$Context.Web.Update()
$Context.ExecuteQuery()

Write-Host "Time Zone successfully updated for $($site) "
}