You can sort related records in relation graph, not only portal. If you sort in graph, you can get value from 1st related record according to the sort order as
Or, there is another function than GetRepetition,
GetNthRecord ( table::field ; num )
num can be count of related record, for "last" record on sort of relation graph.
These 2 forms does nothing with portal if that is evaluated out of portal, get data from relation.
Get(Repetition) has nothing to do with Portals. Portals are related information from another table, where as Repetition are asking for information in your own table.
Your portal is using a relationship. That relationship could have a sort order or not. If it does not have a sort order, the related records would be in creating order through your relationship. In your layout you can then setup if you want the portal records to be sorted or not.
1. You have a sorted relationship. Then you will have all records be sorted on your date field
2. You do NOT have sorted relationship but have it sorted in your portal setup. Then you can have information from first created record though your relationship.
Then you can also use GthNthRecord like user19752 wrote about above.