NickName:timsterc Ask DateTime:2014-07-03T03:54:12

Use different date as UNIX epoch time in Java?

I know that the standard UNIX epoch time is 1/1/70 and I can use this variable to convert current time to UNIX time:

int time = (int) ((System.currentTimeMillis())/1000);

(Note: I know it's normally stored as a 64-bit long but I need it to be 32 bit, hence the cast to int).

My question is - are there any built-in methods in Java where you can pass in a different date as the "epoch date." What I am trying to get is a timestamp in # of seconds since a particular date. I.e. I would like to define January 1, 2015 as MY epoch. And then each future date would be # of seconds since 1/1/2015.

If there's no Java library that provides that capability, any ideas on how to do this? Thanks, Tim

Copyright Notice：Content Author:「timsterc」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/24539885/use-different-date-as-unix-epoch-time-in-java

Answers

Peter Lawrey 2014-07-02T20:04:35

There is no library needed, you just have to use maths.\n\nstatic final SimpleDateFormat SDF = new SimpleDateFormat(\"yyyy/MM/dd HH:mm:ss\");\nstatic final long EPOCH = new Date(2015 - 1900, Calendar.JANUARY, 1).getTime(); // 2015/1/1 in GMT\n\npublic static int secondSinceEpoch() {\n return (int) ((System.currentTimeMillis() - EPOCH) / 1000);\n}\n\npublic static String epochToString(int secsSince2015) {\n return SDF.format(new Date(secsSince2015 * 1000 + EPOCH));\n}\n\npublic static void main(String... ignored) {\n System.out.println(new Date(EPOCH));\n System.out.println(\"Epoch is \" + epochToString(0));\n System.out.println(\"Today is \" + secondSinceEpoch() + \" secs\");\n System.out.println(\"Today is \" + secondSinceEpoch() / 86400 + \" days\");\n}\n\n\nprints\n\nThu Jan 01 00:00:00 GMT 2015\nEpoch is 2015/01/01 00:00:00\nToday is -15732850 secs\nToday is -182 days\n",

Q23 2014-07-02T19:59:31

You'd probably have to roll your own time and date methods, which would basically repackage the standard ones while subtracting the correct number of seconds to the epoch to arrive at your new epoch.\n\nSo, maybe something like\n\npublic class MyEpoch {\n public static final Long MY_EPOCH = //number of seconds betweer 1/1/70 and 1/1/2015\n private Date unixEpochDate;\n //etc\n public Long getMyEpoch() {\n return unixEpochDate.getTime() - MY_EPOCH;\n }\n}\n",