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ViewsSpan in days between two dates misunderstanding
I have a misundertood managing dates in Java when I want to calculate the span in number of days between two dates.
Say we have two different dates:
- Date 1: 1986-01-24
- Date 2: 2017-04-20
Case 1: I have this snippet of code using Dates:
SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
Date dt1 = format.parse("1986-01-24");
Date dt2 = format.parse("2017-04-20");
int intSpanInDays= (int) ((dt2.getTime() - dt1.getTime()) / (1000 * 60 * 60 * 24));
System.out.println("Days between: " + intSpanInDays);
Output 1:
Days between: 11408
Case 2: Snippet of code using Calendar:
SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
GregorianCalendar cal1 = new GregorianCalendar();
cal1.setTime(format.parse("1986-01-24"));
cal1.set(Calendar.HOUR, 0);
cal1.set(Calendar.MINUTE, 0);
cal1.set(Calendar.SECOND, 0);
GregorianCalendar cal2 = new GregorianCalendar();
cal2.setTime(format.parse("2017-04-20"));
cal2.set(Calendar.HOUR, 0);
cal2.set(Calendar.MINUTE, 0);
cal2.set(Calendar.SECOND, 0);
long spanInMillis = cal2.getTimeInMillis() - cal1.getTimeInMillis();
GregorianCalendar cal3 = new GregorianCalendar();
cal3.setTimeInMillis(spanInMillis);
long millisInADay = 1000 * 60 * 60 * 24;
System.out.println("Days between: " + (cal3.getTimeInMillis() / millisInADay));
Output 2:
Days between: 11408
Case 3: Example using a spreadsheet in Excel:
When I use MS Excel to get this span just introducing the given dates and simply substracting, the output is this:
QUESTION
Why is Java calculation code of date missing one day? What is missing or wrong in either case 1 and 2 that does not match the result in case 3?
2 answers
Answer question0
The spreadsheet is taking Daylight Savings into account, and your calculations are naively truncating, and given that there's one more 23-hour day in the interval than 25-hour days, the 23-hour remainder is truncated, yielding a result one day less than the correct answer.
0
JDK 8 largely simplifies these calculations with its new date time API. The same can be done accurately and simply using the below code :
LocalDate date1 = LocalDate.of(1986, 01, 24);
LocalDate date2 = LocalDate.of(2017, 04, 20);
System.out.println(date1.until(date2, ChronoUnit.DAYS));
This automatically takes care of any/all the DST changes, leap years etc. which is mostly missed when trying to do the calculations manually.