Why there is a difference between GregorianCalendar.getTimeInMillis() and GregorianCalendargetTimeInMillis(1970.01.01))

Question

I have tried to compare to Dates the Birthdate and the Current date and then I tried to get the age. But the Current date Funtion begins with another Date than the given date My Code:

GregorianCalendar birthdate = new GregorianCalendar(2001,06,20);
long ms = System.currentTimeMillis() - birthdate.getTimeInMillis();
double years = (((double)ms / 1000)/31536000) ;
System.out.print(years);
// years Shoud be 18 here but it returns 17.3
if (years > 18) {
  // Code Block
}
else{
  System.out.print("to Jung");
}

A year is 365.25 days, not 365. As such, there are 31557600 seconds in a year, not 31536000. — Docteur, Jun 20 '19 at 16:12
@Docteur actually... A year has either 365 or 366 days. And maybe + a second (leap seconds). tl;dr: dates are ugly. Try to use a library that does those calculations for you. — Turing85, Jun 20 '19 at 16:13
it still gives 17.., instead of 18, even if the date ist 19.06 instead of 20.06 — Klaus, Jun 20 '19 at 16:13
@Turing85 Yeah, but one year out of four is 366, hence the 365.25. :-) But yes, use a library, don't use milliseconds to calculate how many years have passed. — Docteur, Jun 20 '19 at 16:15
@Klaus have you tried using `new GeorgianCalendar().getTimeMillis()` instead of `System.getCurrentTimeMillis()`? — Turing85, Jun 20 '19 at 16:15
@Docteur even this is an approximation at best. Every 100th year is not a leap year (even though it should be) and every 400th year is a leap year again... =) — Turing85, Jun 20 '19 at 16:16
new GregorianCalendar() gives the same number as System.getCurrentTimeMillis() — Klaus, Jun 20 '19 at 16:17
when i use new GregorianCalendar(1970,1,1) it should give 0ms but it gives 2678400000ms — Klaus, Jun 20 '19 at 16:18
I can’t reproduce exactly. I get `17.93124654426687` (not 17.3). @Docteur The second linked question explains that. — Ole V.V., Jun 20 '19 at 19:44
I recommend you don’t use `GregorianCalendar`. That class is poorly designed and long outdated. Instead use `LocalDate` from [java.time, the modern Java date and time API](https://docs.oracle.com/javase/tutorial/datetime/). — Ole V.V., Jun 20 '19 at 19:45

score 1 · Answer 1 · answered Jun 20 '19 at 16:27

Check the Constructor on GregorianCalendar.

public GregorianCalendar(int year, int month, int dayOfMonth)

Constructs a GregorianCalendar with the given date set in the default time zone with the default locale.

year - the value used to set the YEAR calendar field in the calendar.

month - the value used to set the MONTH calendar field in the calendar. Month value is 0-based. e.g., 0 for January.

dayOfMonth - the value used to set the DAY_OF_MONTH calendar field in the calendar.

You are creating a July 20th date, which is coincidentally offset by a single month, or approximately 0.08 years.

Thanks so i must change the month to 05 and work with the correct number of Millisekunds in the Year 31.556.926.080ms‬ instead of 31.536.000.000ms — Klaus, Jun 20 '19 at 16:46

score 1 · Answer 2 · answered Jun 20 '19 at 16:35

LocalDate birthdate = LocalDate.of(2001,06,20);
LocalDate currentdate = LocalDate.now();
int years = Period.between(birthdate , currentdate).getYears();
if (years >= 18) {
  // Code
} else {
  System.out.print("Sie sind zu Jung");
}

It Worked like this but I must change the hole code because everything was written with the GeorgianCalender.

Docteur · Answer 3 · 2019-06-20T16:31:15.510

I couldn't tell you why exactly your code, which should technically work, doesn't: there seems to be inaccuracies (leap days?) in the Calendar, or I'm missing something. Thanks to Compass, I know that I was missing something: months are 0-indexed in the constructor for Gregorian Calendar. 06 is July.

However, you shouldn't compare dates like this. If you don't want to use any other library, you can do this:

    GregorianCalendar birthdate = new GregorianCalendar(2001,06,20);
    GregorianCalendar today = new GregorianCalendar();
    birthdate.add(Calendar.YEAR, 18);
    System.out.print(birthdate.getTime().after(today.getTime()));

Why there is a difference between GregorianCalendar.getTimeInMillis() and GregorianCalendargetTimeInMillis(1970.01.01))

3 Answers3