---

The OS System Date

Many OSs are able to update the RTC while the OS is running. The conditions under which this occurs are OS-dependent. For instance, MS-DOS(R) (and similar OSs such as PC-DOS) use a fairly simple technique to detect most day, month, or year rollovers and to update the RTC. The OS periodically polls its internally tracked time and when it detects a rollover at midnight, it passes a request to the BIOS to update the RTC time and date. However, MS-DOS polls only when idle, so it may not have the opportunity to update the RTC if programs are continuously executing.

MS-DOS exhibits a further peculiarity dating back to its roots, which compounds the RTC century rollover problem. For MS-DOS, a significant date marks the "beginning of time;" January 1, 1980, is the earliest date recognized by any version of MS-DOS. This is due to a data reduction technique used to stamp files with the current date, expressed as an offset from the base date 1-1-1980. The technique reduced the date stamp from a total of 23 bits of information (5 bits for the day of month [1-31], 4 bits for the month [1-12], and 14 bits for the year [0-9999]), down to 16 bits. This was accomplished mainly by limiting the year field to 7 bits, and using it as an offset from the year 1980. Under this scheme, any year from 1980 to 2099 can be expressed using offset values ranging from 0 to 119. For instance, the date 1996 is expressed by 16 (1980+16=1996). This technique optimized scarce system resources and also made file date comparisons much easier to implement.

The inability of MS-DOS to recognize dates prior to 1-1-1980 may produce unusual behavior at the century rollover. The most likely scenario involves a system powered off at the time of the century rollover. For instance, on December 23, 1999, a user powers off his system and leaves for vacation. The user returns on January 3, 2000 and powers on the system. The RTC passes the date 1-3-1900 to the OS. Since the earliest date MS-DOS recognizes is 1-1-1980, it interprets 1900 as invalid, and sets the date to its earliest known date, 1-1-1980 (or, in some cases to 1-4-1980). Further, if the PC is networked and the date on the server is likewise incorrect, the local system date will remain out of date.

Current versions (as of August 1996) of the Microsoft(R) Windows(R) 95 and Windows 3.x OSs display the same difficulties with the century rollover as MS-DOS, returning either 1-1-1980 or 1-4-1980 as the system date. The Microsoft Windows NT(R) 3.x OS, while not subject to the MS-DOS inability to recognize dates prior to 1-1-1980, is unable to correct the RTC clock date, returning 1-1-1900 as the system date. In contrast, the Windows NT 4.0 OS is able to override the incorrect RTC date and return the correct system date at the century rollover. Like Windows NT, most major OSs are likely to be upgraded in the next few years to correct the problem with the system date at the century rollover.

In addition to the expected OS upgrades, modifications to the system BIOS will further ensure that most PCs handle the century rollover correctly.

---