I added the system date and time to the sign-on screen using SDA. Much to my surprise, the date was not always accurate. My initial response was, "How could the system date be wrong?" But the system date was always correct. Where else could the date be coming from? Testing the screen with SDA always showed the correct date.
Finally, a call to IBM service provided the surprising answer. The date on the sign-on screen is not the system date but, instead, the date that QINTER was started. Our operators end QINTER prior to the nightly backup and then start QINTER when the backup has finished. This usually happens after midnight-but the process can end before midnight, causing the date on the sign-on screen to be one day behind.