Get-GSCalendarEvent -TimeMin (Get-Date).Date -TimeMax (Get-Date).Date.AddDays(1) -SingleEvents |
  ForEach-Object { $_.End.DateTime - $_.Start.DateTime } |
  Measure-Object -AllStats

Count             : 3
Average           : 1:00:00
Sum               : 3:00:00
Maximum           : 2:00:00
Minimum           : 0:30:00
StandardDeviation : 0:30:00
Property          :

Get-GSCalendarEvent -TimeMin (Get-Date).Date -TimeMax (Get-Date).Date.AddDays(1) -SingleEvents |
  ForEach-Object { $_.End.DateTime - $_.Start.DateTime } |
  ForEach-Object { $s = New-TimeSpan } { $s += $_ } { $s }