Question

Sorry if this is a re-post. I did not find a suitable answer.

Please ignore best practices and use of deprecated apis.

I want to format the following date “Mon May 19 02:16:52 EDT 2003” into “MM/dd/yyyy HH:mm:ss“. Again, I only want to change the output format and this is being executed from my laptop which is in EST.

My Windows Regional Settings are:

Current time zone: Eastern Daylight Time

Timezone: (GMT-05:00) Eastern Time (US & Canada)

Automatically adjust clock for daylight saving changes: Checked

This java code does it:

Date aDate = new Date("Mon May 19 02:16:52 EDT 2003");
SimpleDateFormat sdf = new SimpleDateFormat( "MM/dd/yyyy HH:mm:ss");
System.out.println("Formatted Date: " + sdf.format(aDate));

Output is

Formatted Date: 05/19/2003 02:16:52

Now I change the windows setting to the following (only uncheck the DST setting)

Current time zone: Eastern Daylight Time

Timezone: (GMT-05:00) Eastern Time (US & Canada)

Automatically adjust clock for daylight saving changes: Checked

Output is:

Formatted Date: 05/19/2003 01:16:52

Questions:

1. Why is the output off by an hour?
2. Does java use Windows DST settings even when formatting? I though java maintains its own data for DST settings times for various timezones.

Answer 1

Internally, dates are stored by a long value that represents the number of milliseconds past some epoch. When you clicked off the daylight savings time on you machine, you changed your timezone. So Java is using the same time number with a different timezone and that’s why you have what you have