Solution
Format Date/ Time String VI and Get Date/ Time String VI can fall behind real-time when using older versions of LabVIEW Real-Time (RT) due to limitations in the Real-Time Operating System (RTOS). When the RTOS runs code set for time-critical priority, it ignores its book-keeping routines.
One of the book-keeping tasks of the RTOS (or any OS for that matter) is to keep system time. In LabVIEW RT, this is done with interrupt service requests, which get ignored when the RTOS is busy running time-critical code. This is how the RTOS is able to maintain deterministic and reliable behavior.
Beginning with LabVIEW 8.0, a new timing source has been implemented which does not experience this behavior. Additionally, this timing source allows for a true millisecond timing source for Timed Loops. Hence, using Get Date/ Time in Seconds VI should now keep real-time.