Afaik there is currently no cmdlet in the VI Toolkit that returns the date/time for an ESX server.
So the SDK is the obvious next choice and there are indeed 2 methods to get the time of an ESX server.
Method 1:
On the ServerInstance MO there is a method called CurrentTime.
Method 2:
Since SDK 2.5 there exists a method called QueryDateTime on the HostDateTimeSystem MO.
But apparently both methods fail when called from PowerShell with the message:
Exception calling "CurrentTime" with "0" argument(s): "Value cannot be null. Parameter name: destType"
Any explanation for this behavior would be welcome 
Luckily the ServerInstance MO has a property, called ServerClock, that does return the date and time in UTC.
This script shows you the date/time of an ESX server.
Get-VIServer -Server <your-ESX-server>
# Force load of the assembly
[http://Reflection.Assembly|http://Reflection.Assembly]::LoadWithPartialName("vmware.vim")
$svcRef = new-object VMware.Vim.ManagedObjectReference
$svcRef.Type = "ServiceInstance"
$svcRef.Value = "ServiceInstance"
$serviceInstance = get-view $svcRef
$datetime = $serviceInstance.ServerClock
Write-Host $datetime
The script is attached since the forum SW has a mind of it's own.