1000 * 60 * 60 * 24 * 30 results in a negative number

Question

I'm attempting to calculate 30 days by multiplying milliseconds however the result continually ends up being a negative number for the value of days_30 and I'm not sure why.

Any suggestions are greatly appreciated!

CODE SNIPPET:

// check to ensure proper time has elapsed
                SharedPreferences pref = getApplicationContext()
                        .getSharedPreferences("DataCountService", 0);
                 long days_30 = 1000 * 60 * 60 * 24 * 30;
                 long oldTime = pref.getLong("smstimestamp", 0);
                long newTime = System.currentTimeMillis();
                 if(newTime - oldTime >= days_30){

days_30 value results in: -1702967296

P.S.

 double days_30 = 1000 * 60 * 60 * 24 * 30;
                 double oldTime = pref.getLong("smstimestamp", 0);
                double newTime = System.currentTimeMillis();
                 if(newTime - oldTime >= days_30){

Results in a smaller - but still negative number. -1.702967296E9

Check out [this post](http://stackoverflow.com/a/12758352/1679863) — Rohit Jain, Jun 20 '13 at 18:41

score 29 · Accepted Answer · answered Jun 20 '13 at 18:39

You are multiplying ints together, and overflow occurs because the maximum integer is 2^31 - 1. Only after the multiplications does it get converted to a long. Cast the first number as a long.

long days_30 = (long) 1000 * 60 * 60 * 24 * 30;

or use a long literal:

long days_30 = 1000L * 60 * 60 * 24 * 30;

That will force long math operations from the start.

score 2 · Answer 2 · answered Jun 20 '13 at 18:39

2

   long days_30 = 1L * 1000 * 60 * 60 * 24 * 30;

answered Jun 20 '13 at 18:39

ZhongYu

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score 1 · Answer 3 · answered Jun 20 '13 at 18:40

1

Just change your multiplication to long days_30 = 1000L * 60 * 60 * 24 * 30;

answered Jun 20 '13 at 18:40

Guillaume Darmont

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score 0 · Answer 4 · edited May 23 '17 at 10:32

0

Java has limitations when it comes to primitive data types. If your long or double are too big, then it will overflow into a negative number. Try using the BigInteger class for storing larger numbers.

Check this out:

How does Java handle integer underflows and overflows and how would you check for it?

edited May 23 '17 at 10:32

Community

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answered Jun 20 '13 at 18:42

But I'm Not A Wrapper Class

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A `long` is good enough in this case – Steve Kuo Jun 20 '13 at 18:58
If "Computers have limitations when it comes to numbers", then how would BigInteger help? – Steve Kuo Jun 20 '13 at 18:59
If a `double` goes beyond its range, then `POSITIVE_INFINITY` (or `NEGATIVE_INFINITY`) results, not an overflow. – rgettman Jun 20 '13 at 19:01
I'm sorry for the unclear explanation. Java limits primitive data types. For example, longs and doubles are 64 bits (excluding over head). However, if you use a Java object (or a class that extends from Object), the size can as large as the computer's memory is (or whatever the remaining space is before it crashes lol). – But I'm Not A Wrapper Class Jun 20 '13 at 19:29

1000 * 60 * 60 * 24 * 30 results in a negative number

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