It’s About Time

I recently went down the rabbit hole working with PowerShell, datetime and time zone calculations.  Here are some of the things that I learned:

1. 1. 1. 1. 1. Use [datetimeoffset] instead of [datetime]. [datetimeoffset]  has the daylight savings offset as part of the object. Example:

               [datetimeoffset]$ThisDate = Get-Date -Year 2007 -Month 3 -Day 10  -Hour 0 -Minute 0 -Second 0

            2.  The best way to convert AD times such LastLogonTimeStamp is to use the FromFileTime method:

               #Convert LLTs to date
               $RawLLTS = 132467180950915594
               $oLLTS = [System.DateTimeOffset]::FromFileTime($Rawllts)
               $oLLTS

            3. Getting a list of all TimeZones is easy:

               Function ShowPossibleTimeZones($offsetTime) {
                   $offset = $offsetTime.offset
                   Write-Host ("{0} could belong to the following time zones:" -f $offsetTime.ToString());
                   # Get all time zones defined on local system
                   $timeZones = [TimeZoneInfo]::GetSystemTimeZones()
               
                   # Iterate time zones
                   foreach ($timeZone in $timeZones) {
                       # Compare offset with offset for that date in that time zone
                       if ($timeZone.GetUtcOffset(($offsetTime.DateTime)) -eq $offset ) {
                           Write-Host ("   {0}" -f $timeZone.DisplayName);
                       }
                   }
               }

            4. Converting between time zones is also pretty easy. Here is the list of time zone formats for .NET.

               #Zone ID is standard name, $tf is a dot net time format
               Function Convert-TimeToAnotherZone ($dt, $ZoneID,$tf){
                   $Date = [System.DateTimeOffset]::Parse($dt)
                   $objDate = [System.TimeZoneInfo]::ConvertTimeBySystemTimeZoneId($Date,$ZoneID)
               
                   if ($null -eq $tf ){$tf = 'd'}
                   $objDate.DateTime.IsDaylightSavingTime() | Set-Variable isDT
                   if ($isDT){
                       $zone = ([System.TimeZoneInfo]::FindSystemTimeZoneById($ZoneID)).DaylightName
                   }Else{
                       $zone = $TZID
                   }
                   $objDate.DateTime.tostring($tf) + " $zone"
               } 
               
               #Convert date to another time zone by zone name
               $testDate = [System.TimeZoneInfo]::ConvertTimeBySystemTimeZoneId($ThisDate,'Central Standard Time')
               Convert-TimeToAnotherZone -dt $testDate -ZoneID 'Mountain Standard Time' -tf 'G'
               

            5. Putting this together makes it easy to know what the time is in another time zone:

               [System.DateTimeOffset]$now  = Get-Date
               $PT = [System.TimeZoneInfo]::ConvertTimeBySystemTimeZoneId($Now,'Pacific Standard Time')
               Convert-TimeToAnotherZone -dt $pt -ZoneID 'Mountain Standard Time' -tf 'G'