x86, mrst: Add explanation for using 1960 as the year offset for vrtc

Explain the reason for the apparently odd choice of year offset so we don't
get more questions about it.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
LKML-Reference: <20101117121050.9998.89348.stgit@localhost.localdomain>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

drivers/rtc/rtc-mrst.c

@@ -66,7 +66,11 @@ static inline int is_intr(u8 rtc_intr)
  * rtc_time's year contains the increment over 1900, but vRTC's YEAR
  * register can't be programmed to value larger than 0x64, so vRTC
  * driver chose to use 1960 (1970 is UNIX time start point) as the base,
- * and does the translation at read/write time
+ * and does the translation at read/write time.
+ *
+ * Why not just use 1970 as the offset? it's because using 1960 will
+ * make it consistent in leap year setting for both vrtc and low-level
+ * physical rtc devices.
  */
 static int mrst_read_time(struct device *dev, struct rtc_time *time)
 {