Why is this an issue?

The old, much-derided Date and Calendar classes have always been confusing and difficult to use properly, particularly in a multi-threaded context. JodaTime has long been a popular alternative, but now an even better option is built-in. Java 8’s JSR 310 implementation offers specific classes for:

Class Use for

LocalDate

a date, without time of day, offset, or zone

LocalTime

the time of day, without date, offset, or zone

LocalDateTime

the date and time, without offset, or zone

OffsetDate

a date with an offset such as +02:00, without time of day, or zone

OffsetTime

the time of day with an offset such as +02:00, without date, or zone

OffsetDateTime

the date and time with an offset such as +02:00, without a zone

ZonedDateTime

the date and time with a time zone and offset

YearMonth

a year and month

MonthDay

month and day

Year/MonthOfDay/DayOfWeek/…​

classes for the important fields

DateTimeFields

stores a map of field-value pairs which may be invalid

Calendrical

access to the low-level API

Period

a descriptive amount of time, such as "2 months and 3 days"

Noncompliant code example

Date now = new Date();  // Noncompliant
DateFormat df = new SimpleDateFormat("dd.MM.yyyy");
Calendar christmas  = Calendar.getInstance();  // Noncompliant
christmas.setTime(df.parse("25.12.2020"));

Compliant solution

LocalDate now = LocalDate.now();  // gets calendar date. no time component
LocalTime now2 = LocalTime.now(); // gets current time. no date component
LocalDate christmas = LocalDate.of(2020,12,25);