This is basic programming. All you do is create a function with an input and have it return a value. I am going to assume that the data type of the date value is a DateTime
or castable as one.
function MyDateFormat([DateTime]$date) {
return (Get-Date $date -format 'ddd MMM dd yyyy HH:mm:ss') + ' GMT-000'
}
(Please choose a better name, of course. This is only an example.) Then call your function instead:
@{Name=$some_date_attr;Expression=(MyDateFormat $_.some_date_attr)}
If you really want it in GMT, you may want to make sure it's actually using a UTC time.