Question

Why if I convert a date from milliseconds to days, and then back, from days to milliseconds, this date change?

for example:

SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
Date date = format.parse("2012-06-02");
System.out.println(date);
Long dateAsDays = TimeUnit.MILLISECONDS.toDays(date.getTime());

System.out.println(
    new Date(
        TimeUnit.DAYS.toMillis(dateAsDays)
    ) );

will be printed:

Sat Jun 02 00:00:00 GMT+03:00 2012

Fri Jun 01 03:00:00 GMT+03:00 2012

How I can save the day of the month in this conversion? And why this code is not working properly?

Answer 1

The date becomes less accurate when you get it in days. You are in GMT+3, so 12:00 GMT is 3:00 for you. From the TimeUnit class reference:

convert

public long convert(long sourceDuration,
TimeUnit sourceUnit)

Convert the given time duration in the given unit to this unit. Conversions from finer to coarser granularities truncate, so lose precision. For example converting 999 milliseconds to seconds results in 0. Conversions from coarser to finer granularities with arguments that would numerically overflow saturate to Long.MIN_VALUE if negative or Long.MAX_VALUE if positive.
For example, to convert 10 minutes to milliseconds, use: TimeUnit.MILLISECONDS.convert(10L, TimeUnit.MINUTES)