Joda-Time
As for adding a dependency, I'm afraid the java.util.Date & .Calendar really are so bad that the first thing I do to any new project is add the Joda-Time library. In Java 8 you can use the new java.time package, inspired by Joda-Time.
The core of Joda-Time is the DateTime class. Unlike java.util.Date, it understands its assigned time zone (DateTimeZone). When converting from j.u.Date, assign a zone.
DateTimeZone zone = DateTimeZone.forID( "America/Montreal" );
DateTime dateTimeQuébec = new DateTime( date , zone );
LocalDate
One way to verify if two date-times land on the same date is to convert to LocalDate objects.
That conversion depends on the assigned time zone. To compare LocalDate objects, they must have been converted with the same zone.
Here is a little utility method.
static public Boolean sameDate ( DateTime dt1 , DateTime dt2 )
{
LocalDate ld1 = new LocalDate( dt1 );
// LocalDate determination depends on the time zone.
// So be sure the date-time values are adjusted to the same time zone.
LocalDate ld2 = new LocalDate( dt2.withZone( dt1.getZone() ) );
Boolean match = ld1.equals( ld2 );
return match;
}
Better would be another argument, specifying the time zone rather than assuming the first DateTime object’s time zone should be used.
static public Boolean sameDate ( DateTimeZone zone , DateTime dt1 , DateTime dt2 )
{
LocalDate ld1 = new LocalDate( dt1.withZone( zone ) );
// LocalDate determination depends on the time zone.
// So be sure the date-time values are adjusted to the same time zone.
LocalDate ld2 = new LocalDate( dt2.withZone( zone ) );
return ld1.equals( ld2 );
}
String Representation
Another approach is to create a string representation of the date portion of each date-time, then compare strings.
Again, the assigned time zone is crucial.
DateTimeFormatter formatter = ISODateTimeFormat.date(); // Static method.
String s1 = formatter.print( dateTime1 );
String s2 = formatter.print( dateTime2.withZone( dt1.getZone() ) );
Boolean match = s1.equals( s2 );
return match;
Span of Time
The generalized solution is to define a span of time, then ask if the span contains your target. This example code is in Joda-Time 2.4. Note that the "midnight"-related classes are deprecated. Instead use the withTimeAtStartOfDay method. Joda-Time offers three classes to represent a span of time in various ways: Interval, Period, and Duration.
Using the "Half-Open" approach where the beginning of the span is inclusive and the ending exclusive.
The time zone of the target can be different than the time zone of the interval.
DateTimeZone timeZone = DateTimeZone.forID( "Europe/Paris" );
DateTime target = new DateTime( 2012, 3, 4, 5, 6, 7, timeZone );
DateTime start = DateTime.now( timeZone ).withTimeAtStartOfDay();
DateTime stop = start.plusDays( 1 ).withTimeAtStartOfDay();
Interval interval = new Interval( start, stop );
boolean containsTarget = interval.contains( target );
java.time
Java 8 and later comes with the java.time framework. Inspired by Joda-Time, defined by JSR 310, and extended by the ThreeTen-Extra project. See Tutorial.
The makers of Joda-Time have instructed us all to move to java.time as soon as is convenient. In the meantime Joda-Time continues as an actively maintained project. But expect future work to occur only in java.time and ThreeTen-Extra rather than Joda-Time.
To summarize java.time in a nutshell… An Instant is a moment on the timeline in UTC. Apply a time zone (ZoneId) to get a ZonedDateTime object. To move off the timeline, to get the vague indefinite idea of a date-time, use the "local" classes: LocalDateTime, LocalDate, LocalTime.
The logic discussed in the Joda-Time section of this Answer applies to java.time.
The old java.util.Date class has a new toInstant method for conversion to java.time.
Instant instant = yourJavaUtilDate.toInstant(); // Convert into java.time type.
Determining a date requires a time zone.
ZoneId zoneId = ZoneId.of( "America/Montreal" );
We apply that time zone object to the Instant to obtain a ZonedDateTime. From that we extract a date-only value (a LocalDate) as our goal is to compare dates (not hours, minutes, etc.).
ZonedDateTime zdt1 = ZonedDateTime.ofInstant( instant , zoneId );
LocalDate localDate1 = LocalDate.from( zdt1 );
Do the same to the second java.util.Date object we need for comparison. I’ll just use the current moment instead.
ZonedDateTime zdt2 = ZonedDateTime.now( zoneId );
LocalDate localDate2 = LocalDate.from( zdt2 );
Use the special isEqual method to test for the same date value.
Boolean sameDate = localDate1.isEqual( localDate2 );
