"Timestamp" format change

What I would like to do is instead of displaying this:

$obj = $metric.CounterSamples | Select-Object -Property Path, CookedValue, Timestamp;

I would like it to do this:

$obj = $metric.CounterSamples | Select-Object -Property Path, CookedValue, (Timestamp formatted like this yyyy/mm/dd:hh:mm:ss;

so change the "Timestamp" date and time format from it's default to universal.