Decode an Azure JWT Token and find expiry time in local timezone

Decode Azure JWT Token.ps1

function Parse-JWTtoken {
    [cmdletbinding()]
    param([Parameter(Mandatory = $true)][string]$token)
 
    #Validate as per https://tools.ietf.org/html/rfc7519
    #Access and ID tokens are fine, Refresh tokens will not work
    if (!$token.Contains(".") -or !$token.StartsWith("eyJ")) { Write-Error "Invalid token" -ErrorAction Stop }
 
    #Header
    $tokenheader = $token.Split(".")[0].Replace('-', '+').Replace('_', '/')
    #Fix padding as needed, keep adding "=" until string length modulus 4 reaches 0
    while ($tokenheader.Length % 4) { Write-Verbose "Invalid length for a Base-64 char array or string, adding ="; $tokenheader += "=" }
    Write-Verbose "Base64 encoded (padded) header:"
    Write-Verbose $tokenheader
    #Convert from Base64 encoded string to PSObject all at once
    Write-Verbose "Decoded header:"
    [System.Text.Encoding]::ASCII.GetString([system.convert]::FromBase64String($tokenheader)) | ConvertFrom-Json | fl | Out-Default
 
    #Payload
    $tokenPayload = $token.Split(".")[1].Replace('-', '+').Replace('_', '/')
    #Fix padding as needed, keep adding "=" until string length modulus 4 reaches 0
    while ($tokenPayload.Length % 4) { Write-Verbose "Invalid length for a Base-64 char array or string, adding ="; $tokenPayload += "=" }
    Write-Verbose "Base64 encoded (padded) payoad:"
    Write-Verbose $tokenPayload
    #Convert to Byte array
    $tokenByteArray = [System.Convert]::FromBase64String($tokenPayload)
    #Convert to string array
    $tokenArray = [System.Text.Encoding]::ASCII.GetString($tokenByteArray)
    Write-Verbose "Decoded array in JSON format:"
    Write-Verbose $tokenArray
    #Convert from JSON to PSObject
    $tokobj = $tokenArray | ConvertFrom-Json
    Write-Verbose "Decoded Payload:"
    
    return $tokobj
}

function Convert-UnixTime {     
    Param([Parameter(Mandatory = $true)][int32]$unixDate)          
    $orig = (Get-Date -Year 1970 -Month 1 -Day 1 -hour 0 -Minute 0 -Second 0 -Millisecond 0)         
    $timeZone = Get-TimeZone     
    $utcTime = $orig.AddSeconds($unixDate)    
    $localTime = $utcTime.AddHours($timeZone.BaseUtcOffset.Hours)     # Return local time    
    return $localTime 
}

$azureToken = Parse-JWTtoken($Global:accesstoken)
$tokenExpires = Convert-UnixTime($azureToken.exp)
$tokenExpires