xact_start time set to very old value

Hi Team,
We have observed that xact_start time is null for some period and suddenly it populates to very old value. We have a query which runs every minute to report any transactions running for long time. That query reports nothing continuously for a minute before and suddenly it reports one transaction which is running for more than one day. Is there something wrong here in populating xact_start time in pg_stat_activity? Shall we go with any other approach to detect long running transaction in database? Can someone guide on this?

I went through a thread - /messages/by-id/20140424101827.2714.39486@wrigleys.postgresql.org but not sure where it concluded.
Appreciate your response.
We are using AWS RDS 11.4.
Regards,-Virendra