Hey @kenken,
Exactely what I was looking for 
Plus: Now I also know what those mysterious cronjob-things actually do.
Many thanks!
For the sake of completion I’m posting my solution here:
SyncedCron.add({
name: 'Remove old Entries (90 Days)',
schedule: function(parser) {
return parser.text('every 8 hours');
},
job: function() {
var today = new Date();
var targetDate = new Date();
targetDate.setDate(today.getDate() - 90);
targetDate.setHours(0);
targetDate.setMinutes(0);
targetDate.setSeconds(0);
// Remove matchng Documents
Entries.remove({createdAt: {$lt: targetDate}});
}
});
// Start Cronjobs
SyncedCron.start();
If anyone finds an error in my approach please let me know.
Thanks.