Get Dates that match a periodicity and get number of sequence

Question

I need to know if a date match a periodicity, for example, periodicity is 1 hour, and date that user gives is 13/09/2021 23:00, the inicial that my java code should take is 13/09/2021 00:00 and check how many times have to add 1 hour to get the date 13/09/2021 23:00.

The idea now is made a loop and add 1hour to the date and save in an array, then check if the date is inside the array and the position. Is there any other way?

Answer 1

If I understand your question correctly, you just want to calculate how many hours there are between two dates. For that, it's cleaner to use the built-in java.time classes. You can read the two dates into LocalDateTime objects and calculate the time span between them with ChronoUnit.HOURS:

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd/MM/yyyy HH:mm");
LocalDateTime start = LocalDateTime.parse("13/09/2021 00:00", formatter);
LocalDateTime end = LocalDateTime.parse("13/09/2021 23:00", formatter);
long hours = ChronoUnit.HOURS.between(start, end);

The result will be 23.

For various other units (minutes for example), there's ChronoUnit.MINUTES. Have a look at the documentation. There are a lot of different units to choose from.

Answer 2

I am adding a couple of minor refinements compared to the correct answer by QBrute.

1. The periodicity can be any amount of time in hours, minutes and seconds.

2. I am taking time zone into account so we also get correct results across summer time transitions (spring forward and fall back) and other time anomalies.

3. If there isn’t a whole number of periodicities, I am rounding up to be sure to have at least enough.

   ZoneId userTimeZone = ZoneId.of("Africa/Dar_es_Salaam");
   
   Duration periodicity = Duration.ofMinutes(5);
   ZonedDateTime userTime = ZonedDateTime.of(2021, 9, 13, 23, 0, 0, 0, userTimeZone);
   ZonedDateTime initialTime = ZonedDateTime.of(2021, 9, 13, 0, 0, 0, 0, userTimeZone);
   
   Duration timeBetween = Duration.between(initialTime, userTime);
   long numberOfPeriodicities = timeBetween.dividedBy(periodicity);
   // Has truncation occurred?
   if (initialTime.plus(periodicity.multipliedBy(numberOfPeriodicities)).isBefore(userTime)) {
       // Need one more periodicity
       numberOfPeriodicities++;
   }
   
   System.out.println(numberOfPeriodicities);
   

Output is:

276

If you want a periodicity of 7.5 minutes, specify Duration.ofMinutes(7).plusSeconds(30). The Duration.dividedBy(Duration) method that I am using was introduced in Java 9.

Link: Oracle tutorial: Date Time explaining how to use java.time.