when property Null safety
Specifies the timestamp of the notification.
To control whether the timestamp is shown in the notification, use showWhen.
The timestamp is expressed as the number of milliseconds since the
"Unix epoch" 1970-01-01T00:00:00Z (UTC). If it's not specified but a
timestamp should be shown (i.e. showWhen is set to
then Android will default to showing when the notification occurred.
final int? when;