I think this is simply the date in which the entry was recorded in the ledger/journal.
The following is from the links below:
Valutadag eller Reskontradag
Om du vill söka transaktioner per en viss valutadag eller reskontradag så kan du ange datumet här. Du kan ange ett datum upp till 14 månader tillbaka i tiden. Datumet kan ligga såväl innanför som utanför den period du angivit för bokföringsdatum. Markera om angivet datum avser valutadag eller reskontradag.

Journal Entries
[...] The first step in the accounting process is to analyze each transaction and identify what effect it has on the accounts. After making this determination, an accountant enters the transactions in chronological order into a journal, a process called journalizing the transactions. Although many companies use specialized journals for certain transactions, all businesses use a general journal.