LinuxQuestions.org - sleep & localtime query

Hi,
I work on HPUX application and i had a query regarding the sleep & localtime system call in HPUX.
Here is the code that we have :

const uint32_t WAKE_INTERVAL_SEC(30*60);
while (true) {

// Find out what time it is
time(&currentTime);

// Thread to align wakeup with clock 30 minute i.e if it is currently 9:20 wakeup after 10 minutes,
// If it is currently 9:05, wake up after 25 minutes.
// Calculate remaining number of seconds in this half hour.
sleepTime = WAKE_INTERVAL_SEC - (currentTime % WAKE_INTERVAL_SEC);
while (1) {

sleep(sleepTime);

// Do regular work
tptr = localtime(&currentTime);

if (12 == tptr->tm_hour && tptr->tm_min <WAKE_INTERVAL_MIN)
{
// Noon work to be done
}
}

Expected Behaviour:
As refered in above code, we expect that every half an hour, some work to be done and if that time happens to be between 12 and 12:30am then we also do noon work.

Actual Behaviour:
Most of the time it works as expected. But some times, sleep seems to exit a few seconds before 12:30 and so the logic fails and does the noon work. Ideally i would expect that noon work condition to fail, since thread has woke up at 12:30 and hence it cannot satisify noon work condition. ( Trace in syslog indicates that regular work has started at 12:30, but logging itself would have taken some time and i suspect sleep could have woken up early)

Question:
- Can sleep wake up before the schedule time ( other than SIGALRM) ? Are there any other conditions where sleep could wake up before scheduled time?
- Can localtime used for comparison return wrong time ?
- Is it possible that there are some time adjustments done internally(may be for internal clock drifting etc) which results in sleep waking up a second earlier than expected or local time doing some kind of adjustment for some reason?

Any inputs in this regards are appreciated.

Thanks & Regards,
nsv