Java：如何在不使用日历的情况下操作日期以添加天数或月数？

Question

I have a simple function that compares two dates, if the two dates are equal - I want to add 7 days to one of the dates, or add a month depending on the scenario. I have two dates formatted using simpleDateFormat that are converted to date from a string, then I compare them like so

Date dateExpired = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'").parse(expDate);
Date dateEffective = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'").parse(effDate);

Comparison method:

if(dateExpired.before(dateEffective) || dateExpired.equals(dateEffective)) {
    System.out.println("Same date for effective and expired");
    
}

Inside my comparison, I want to make the update. Can anyone tell me how to do this?

Answer 1

Never use Calendar , part of the terrible date-time classes that were years ago supplanted by the modern java.time classes defined in JSR 310.

Never put single-quote marks around the Z in your date-time formatting pattern. That letter indicates an offset-from-UTC of zero hours-minutes-seconds. Your quotes are discarding valuable information.

Your input strings are apparently in standard ISO 8601 format. The java.time classes use the standard formats by default when when parsing/generating text. So no need to specify a formatting pattern.

Parse your inputs as Instant objects to represent a moment as seen in UTC.

Instant expirationUtc = Instant.parse( x ) ;
Instant effectiveUtc = Instant.parse( y ) ;

For any given moment, the date varies around the globe by time zone. It can be “tomorrow” in Tokyo Japan while simultaneously “yesterday” in Toledo Ohio US.

So you must adjust your inputs to the time zone by which you want to perceive the date.

ZoneId z = ZoneId.of( "America/Edmonton" ) ; 
ZonedDateTime expiration = expirationUtc.atZone( z ) ;
ZonedDateTime effective = effectiveUtc.atZone( z ) ;

Test for equality of dates. Extract the date portion to compare.

if( expiration.toLocalDate().isEqual( effective.toLocalDate() ) ) {…}

Represent a span of time using Period class, for your month or week.

Period period = Period.ofWeeks( 1 ) ;
ZonedDateTime later = expiration.plus( period ) ;