Add 30 minutes to the current time-CodePudding

I have written the code below but if the current date-time is 2022-07-03 09:48:05.448 and I add 30 minutes, my response returns 2022-07-03 09:79:05.448.

But minutes can never be 79, it is supposed to move to the hours instead...

public static String getExpiryDate(int additionalMinutesToCurrentMinute) {
    LocalDateTime now = LocalDateTime.now();
    int year = now.getYear();
    int month = now.getMonthValue();
    int day = now.getDayOfMonth();
    int hour = now.getHour();
    int minute = now.getMinute()   additionalMinutesToCurrentMinute;
    int second = now.getSecond();
    int millis = now.get(ChronoField.MILLI_OF_SECOND); // Note: no direct getter available.

    String expiryDateAndTime = String.format("%d-d-d d:d:d.d", year, month, day, hour, minute, second, millis);
    return expiryDateAndTime;
}

CodePudding user response：

Explanation

The reason your code does not work as expected is because you are not involving javas Date/Time API at all in your "math".

Your adding the minutes with plain int-arithmetic

int minute = now.getMinute()   additionalMinutesToCurrentMinute;

and then you use plain string formatting

String.format("%d-d-d d:d:d.d", year, month, day, hour, minute, second, millis);

Nothing in this chain is "clever" and knows about date/time specifics.

Solution

You have to involve the Date/Time API for your math, then it will be clever and correctly adjust the hours as well. Fortunately, there is a method in LocalDateTime already that does what you want:

LocalDateTime expirationTime = LocalDateTime.now().plusMinutes(30);

and that is pretty much all you need.

For the formatting part, either roll with the default representation:

return expirationTime.toString();

or use a DateTimeFormatter:

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy MM dd HH:mm:ss.AAA");

and then

return expirationTime.format(formatter);

Notes

Instant

You are actually using the incorrect type for an expiration time. Using LocalDateTime will result in your application failing under certain situations. For example if your computer moves across countries, or your government decides to change its timezone. Or when DST hits (summer vs winter time) or leap seconds are added and more...

The correct type would be Instant, which represents a single moment on the timeline, without interpretation of clock-time or calendar-dates.

The API is the same, so you can just use it the same way.

That said, your method should also return Instant and not a String. Keep the clever date/time type as long as possible, dont go to something as low level and raw as a string.

public static Instant getExpiryDate(int additionalMinutes) {
  return Instant.now()
    .plus(additionalMinutes, ChronoUnit.MINUTES);
}

Design

Design-wise it would be better if the method would not even take int additionalMinutes but also the unit. Otherwise the call-site is hard to read for users:

getExpiryDate(30) // 30 what? minutes? seconds? days?

with the unit, it would be easier to read and harder to misunderstand

getExpiryDate(30, ChronoUnit.MINUTES)

At which point one could argue that the method is kinda obsolete now.

CodePudding user response：

Instead of editing the amount of minutes manually, try using the plusMinutes method on your LocalDateTime like so:

LocalDateTime now = LocalDateTime.now();
LocalDateTime then = now.plusMinutes(30);

This way, the class should increase the hour for you once it passes 60 minutes.