Likely timezone rounding. Happens also on other airline sites. For example, on LH's Miles & More site, it'll always adjust your date of birth to one day earlier than you entered, which is rather distressing at ticketing (no matter how much you adjust it, once you save it it shows it a day earlier again)...but it's just display, as it adjusts from US timezone to EU timezone. Seems like just another bug that Dullta has rolled out, staying true to its firmest belief to never, ever do any Q&A testing before making changes to its passenger facing systems.