Java: date & time relative to current (server) time

Question

We're using MySQL to store some dates. I'm looking to show these as relative time periods in the user interface: 2 hours ago, 3 days ago etc. (Like Twitter does for the updates for example)

Is there a well-known way to accomplish this, or should I get creative with it?

Just to be clear, I want to transform: 07/26/2009 12:20 -> '2 days ago'

Answer 1

I think the most common solution is to convert it to unix timestamps (or equivalent, milliseconds in Java normally), take the difference and start dividing away.

time = now - then;
time /= 1000; /* if milliseconds... */
seconds = time % 60; time /= 60;
minutes = time % 60; time /= 60;
hours   = time % 60; time /= 60;
days    = time % 24; time /= 24;
weeks   = time % 7; time /= 7;

or months (although, then it starts to get tricky...), or whatever you want to use.

Have fun.

Answer 2

My advice is run your server in UTC and then use JodaTime to do any date arithmetic or conversion between timezones.

There's considerably more to date arithmetic than meets the eye once you factor in things like DSL, leap-seconds, convention changes, etc and it's really the last thing you want to be doing yourself.

Answer 3

The question is rather vague.

In Java, using JodaTime Date API;

3 days ago:

DateTime date = new DateTime();
DateTime threeDA = date.plusDays(-3);
int daysBetween = Days.daysBetween(dbDate, threeDA).getDays();
int monthsBetween = Months.monthsBetween(dbDate, threeDA).getMonths();

or you could use the JodaTime Period/Duration objects.

In MySQL, use a built-in MySQL Date Function e.g.:

SELECT SUBTIME(SYSDATE(),'3'); -- untested, no MySQL to hand
SELECT SUBTIME('2007-12-31 23:59:59.999999','3 0:0:0.000000');

For date differences:

SELECT DATEDIFF(columnname, SYSDATE()); -- Days since
SELECT TIMEDIFF(columnname, SYSDATE()); -- Time since

In Java, using Gregorian Calendar:

GregorianCalendar threeDA = new GregorianCalendar();
threeDA.add(GregorianCalendar.DAY_OF_YEAR, -3);

Answer 4

If you can use java for the conversion, have a look at the Joda library.

Answer 5

I would take a look at the Joda library for performing this type of date-time arithmetic. For example, you could create a Joda Duration and then convert it to a Period, giving you access to numerous useful methods:

ResultSet rs = ...
Date dbDate = rs.getDate("Date"); // Get stored time in database.
long serverTime = System.currentTimeMillis(); // Get current server time.

// Compute absolute difference between two time-stamps.
Duration duration = new Duration(Math.abs(serverTime - dbDate.getTime()));

// Convert to period and make use of getHours(), getMinutes(), etc for display purposes.
Period period = duration.toPeriod();

System.err.println("Hours: " + period.getHours());
System.err.println("Minutes: " + period.getMinutes()); // etc.

Answer 6

On the MySQL side:

CONVERT('date-time-value-here', DATETIME) - NOW()

Answer 7

The Java standard API method for date calculations is Calendar.add() (which also takes negative parameters).

Answer 8

As I understand your problem, the "Human Time" class is a solution.

check Date Formatting and Parsing for Humans in Java with HumanTime .