You could write something like this
Function Get-LocalTime($UTCTime)
{
$strCurrentTimeZone = (Get-WmiObject win32_timezone).StandardName
$TZ = [System.TimeZoneInfo]::FindSystemTimeZoneById($strCurrentTimeZone)
$LocalTime = [System.TimeZoneInfo]::ConvertTimeFromUtc($UTCTime, $TZ)
Return $LocalTime
}
$sessions = Get-NAVServerSession -ServerInstance NAV2016
foreach($session in $sessions)
{
Write-Output Get-LocalTime($session.LoginDateTime)
}