Date Pattern starting from 12 hours

Question

I'm trying to convert two dates into this pattern: yyyy-MM-dd HH:mm:ss.SSS.

I need to retrieve two dates:

• Last year, same month, day 1, 00:00:00.000
• Last day of previous month, 23:59:59.999

So, as of today, I need these two values:

• 2022-11-01 00:00:00.000
• 2023-10-31 23:59:59.999

The problem is: I get 2022-11-01 12:00:00.000.

This is the code that generates last year's date.

private String getLastYear() {
    DateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
    Date date = new Date();
    Calendar c = Calendar.getInstance();
    c.setTime(date);
    c.add(Calendar.YEAR, -1);
    c.set(Calendar.DAY_OF_MONTH, 1);
    c.set(Calendar.HOUR, 0);
    c.set(Calendar.MINUTE, 0);
    c.set(Calendar.SECOND, 0);
    c.set(Calendar.MILLISECOND, 0);
    return format.format(c.getTime());
}

If I try adding one hour, the output will be: 2022-11-01 13:00:00.000.

Also, I can't retrieve the last day of the previous month. As I have read here, all I keep getting is the first day of the next month.

Here's the code.

private String getPreviousMonth() {
    DateFormat format = new SimpleDateFormat(DATE_PATTERN);
    Calendar c = Calendar.getInstance();
    Date date = new Date();
    c.setTime(date);
    c.add(Calendar.MONTH, -1);
    c.set(Calendar.DAY_OF_MONTH, c.getActualMaximum(Calendar.DAY_OF_MONTH));
    c.set(Calendar.HOUR, 23);
    c.set(Calendar.MINUTE, 59);
    c.set(Calendar.SECOND, 59);
    c.set(Calendar.MILLISECOND, 999);
    return format.format(c.getTime());
}

There is something I'm missing. Any help is highly appreciated!

Answer 1

You say any help is highly appreciated, so here's an example that uses java.time, which uses today as base and calculates the two desired results as follows:

First of current month in the last year

• subtract a year from today (2023-11-17 ⇒ 2022-11-17)
• take the first of that month (2022-11-17 ⇒ 2022-11-01)
• involve the start of the day (2022-11-01 ⇒ 2022-11-01 00:00:00.000)

Last day of last month at the end of the day

• subtract a month from today (2023-11-17 ⇒ 2023-10-17)
• go to the last day of that month (2023-10-17 ⇒ 2023-10-31)
• append the maximum time of day (2023-10-31 ⇒ 2023-10-31 23:59:59.999)

public static void main(String[] args) {
    LocalDateTime lastYearSameMonthFirstDay
                        // today (without time of day)
                        = LocalDate.now()
                                   // subtract 1 from the year
                                   .minusYears(1)
                                   // go to the first day of month 
                                   .with(TemporalAdjusters.firstDayOfMonth())
                                   // append the start of the day
                                   .atStartOfDay();
    
    LocalDateTime lastDayOfPreviousMonth
                        // today (date only as above)
                        = LocalDate.now()
                                   // subract 1 from the month value
                                   .minusMonths(1)
                                   // go to the last day of that month
                                   .with(TemporalAdjusters.lastDayOfMonth())
                                   // append the end of the day
                                   .atTime(LocalTime.MAX);
    // prepare a formatter for the output
    DateTimeFormatter dtf = DateTimeFormatter.ofPattern("uuuu-MM-dd HH:mm:ss.SSS");
    // and print both LocalDateTimes formatted as desired
    System.out.println(lastYearSameMonthFirstDay.format(dtf));
    System.out.println(lastDayOfPreviousMonth.format(dtf));
}

Output (execution date 2023-11-17):

2022-11-01 00:00:00.000
2023-10-31 23:59:59.999

Answer 2

"... I need to retrieve two dates:

• Last year, same month, day 1, 00:00:00.000
• Last day of previous month, 23:59:59.999 ..."

Here is an example.

import static java.util.Calendar.*;

Calendar a, b;
a = GregorianCalendar.getInstance();
a.set(YEAR, a.get(YEAR) - 1);
a.set(DAY_OF_MONTH, 1);
a.set(HOUR, 0);
a.set(MINUTE, 0);
a.set(SECOND, 0);
a.set(MILLISECOND, 0);
b = GregorianCalendar.getInstance();
b.add(MONTH, -1);
b.set(DAY_OF_MONTH, b.getActualMaximum(DAY_OF_MONTH));
b.set(HOUR_OF_DAY, 23);
b.set(MINUTE, 59);
b.set(SECOND, 59);
b.set(MILLISECOND, 999);

"... Also, I can't retrieve the last day of the previous month. As I have read here, all I keep getting is the first day of the next month. ..."

Use the getActualMaximum method.

"... I'm trying to convert two dates into this pattern: yyyy-MM-dd HH:mm:ss.SSS. ..."

Optionally, you can use basic format specifiers here.
There is a full list in the Formatter JavaDoc.

System.out.printf("%tF %<tT.%<tL%n", a.getTime());
System.out.printf("%tF %<tT.%<tL%n", b.getTime());

Output

2022-11-01 00:00:00.000
2023-10-31 23:59:59.999