In version 16, the way the Date/Time is calculated from the controller clock was changed.
The WALLCLOCKTIME object is now Universal Coordinated Time, more commonly called "Greenwich Mean Time", or "Zulu Time" if you have a crewcut and good posture.
There are new related objects, including LocalDateTime, which has the option of implementing Daylight Saving Time.
If you're on the East Coast of the USA and posting after 8 PM, it's already after midnight GMT so the Day value of WALLCLOCKTIME will be 1 greater than you expect, but LOCALDATETIME will have the expected Day value.
This change is well described in the v16 Release Notes and in RA Knowledgebase
Article ID 37953 (TechConnect).