References for implementing calendar functionality in an embedded system?

I have an embedded system that currently keeps track of seconds until an event is supposed to occur using a real-time clock driven by a watch crystal.

Now it needs to keep track of the actual date and time. So, I need to be able to calculate the day, month, year, hour, minute and sec开发者_如何学编程ond from a start date/time and offset in seconds.

Could anyone point me in the right direction for taking into account leap years, daylight savings time (DST) and other complications?

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Hardware solutions are not an option as this feature is being added to an existing product. An RTC peripheral is integrated into the MCU chosen for the next generation device.

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The C Snippets archive has some date and time functions. Update: unfortunately, the C snippets archive is now defunct. I have updated the link to point to the web archive of the page.

See also "Julian day", Wikipedia, which includes formulas for Julian date calculation.

A "julian date calculation" Google search should uncover more if you want to search further.

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Calendar code can be a bit complex - if the C runtime library you're using doesn't have such support built-in (and some way to integrate your clock counter to it) you might consider looking at the code in P.J. Plauger's "The Standard C Library" and adapting it to your needs.

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I'm bored, couldn't resist trying a solution. Here's a prototype in ruby - should be clear enough to translate to C.

Given offset and a start date stored as: Baseyear, Baseday, Basesec where day 0 = Jan1, you can calculate the date as

#initialize outputs
year= Baseyear
day = Baseday
sec = Basesec+offset

#days & seconds remaining in the current year
is_leap = is_leap_year(year)
days_remaining = 365+(is_leap ? 1 : 0) - day
secs_remaining = SEC_PER_DAY*days_remaining

#advance by year
while (sec>=secs_remaining)
  sec-=secs_remaining
  year+=1
  is_leap = is_leap_year(year)
  days_remaining = 365+(is_leap ? 1 : 0)
  secs_remaining = SEC_PER_DAY*days_remaining
  day=0 
end

#sec holds seconds into the current year, split into days+seconds
day += sec / SEC_PER_DAY
day = day.to_i #cast to int
sec %= SEC_PER_DAY

#lookup month
for i in (0..11)
  dpm = DAYS_PER_MONTH[i] # =[31,28,31,30,...]
  if (i==1 && is_leap) 
   dpm+=1
  end
  if day < dpm
   month = i
   break
  else
    day-=dpm
  end
end

day+=1 #1-based
hour = sec/3600
min = (sec%3600)/60
sec = sec%60
puts "%s %d, %d @ %02d:%02d:%02d" % [MONTHNAME[month],day,year, hour, min, sec]

It should be easy to add a check that the day is between the begin and end days for DST in the current locale, and adjust the hour accordingly.

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The following function determines whether a given year is a leap year:

bool is_leap_year(int year)
{
    return ((0 == year % 400) || ((0 == year % 4) && (0 != year % 100)));
}