AN923 Datasheet, PDF (5/7 Pages) STMicroelectronics – Managing century information using serial real-time clocks

AN923
Support for leap year
Support for leap year
Leap year occurs every four years, in years which are multiples of 4. For example, 2012 was
a leap year. An exception to that is any year which is a multiple of 100. For example, the
year 2100 is not a leap year. A contradiction to that is that years which are multiples of 400
are indeed leap years. Hence, while 2100 is not a leap year, 2400 is.
During any year which is a multiple of 4, ST RTC and TIMEKEEPER devices will
automatically insert leap day, February 29. Therefore, the application software must correct
for this during the exception years (2100, 2200, etc.) as noted above.
M41TC8025 serial RTC and leap year
The M41TC8025 handles leap year differently than all other ST clock devices. It treats all
years ending in 00 as non-leap years. Thus, the years 2100, 2200, 2300, 2400, et cetera,
all skip leap day. For this device, the application software must insert leap day in the
appropriate years. For example, the software will need to insert leap day in 2400, but not
2100, 2200 nor 2300, as noted above.
For any other year which is a multiple of 4, the M41TC8025 properly inserts leap day just
like ST's other RTC devices do.
For more information about ST's TIMEKEEPER NVRAM's and serial real-time clocks,
please contact a local ST sales office or visit www.st.com.
DocID005227 Rev 3
5/7
7