如何在kotlin中获取重置时间为00:00的当前日期

Question

Hey I am using calendar instance to get current date and time.

private fun getCurrentCalendar() { Calendar.getInstance() }

Output:- Wed Jul 20 21:45:52 GMT+01:00 2022

If I want to reset time I need to use this function

fun resetCalendarTime(calendar: Calendar) {
     calendar.set(Calendar.HOUR_OF_DAY, 0)
     calendar.set(Calendar.MINUTE, 0)
     calendar.set(Calendar.SECOND, 0)
     calendar.set(Calendar.MILLISECOND, 0)
}

it will give this

Wed Jul 20 00:00:00 GMT+01:00 2022

My question is there any efficient way of doing that or this is fine in android?

Answer 1

Caveat: Java syntax shown here, as I have not yet learned Kotlin.

tl;dr

LocalDate.now( zoneId ).atStartOfDay( zoneId )

Details

The terrible java.util.Calendar class was years ago supplanted by the modern java.time classes defined in JSR 310. Never use either Date class, Calendar , SimpleDateFormat , or any other legacy date-time class found outside the java.time package.

On JVM

If you are running Kotlin on a JVM , use java.time classes.

To get the current moment as seen in a particular time zone, use ZonedDateTime .

ZoneId z = ZoneId.of( "Asia/Tokyo" ) ;
ZonedDateTime zdt = ZonedDateTime.now( z ) ;

If you want to use the JVM's current default time zone:

ZoneId z = ZoneId.systemDefault() ;

Apparently you want the first moment of the day. Your Question makes the mistake of assuming that time is always 00:00. Actually, on some dates in some time zones, the first moment occurs at a different time such as 01:00.

👉 So let java.time determine the first moment of the day. Call LocalDate#atStartOfDay .

A LocalDate represents a date-only value, without time-of-day, and without time zone or offset. Calling atStartOfDay returns a ZonedDateTime , like that seen above.

LocalDate today = LocalDate.now( z ) ;
ZonedDateTime startOfToday = today.atStartOfDay( z ) ;

Here is an example of a day starting at 1 AM.

ZoneId z = ZoneId.of( "Asia/Amman" ) ;
LocalDate ld = LocalDate.of( 2021 , 3 , 26 ) ;
ZonedDateTime zdt = ld.atStartOfDay( z ) ;

See this code run live at Ideone.com . Notice the time is 01:00.

2021-03-26T01:00+03:00[Asia/Amman]

To generate text in standard ISO 8601 format wisely extended to append the name of the time zone in square brackets, merely call ZonedDateTime#toString . See example result quoted directly above this paragraph.

To generate text in other formats, use DateTimeFormatter , and optionally DateTimeFormatterBuilder . To automatically localize, use DateTimeFormatter#ofLocalizedDateTime .

All of this has been covered many many times on Stack Overflow. Search to learn more.

Off JVM

I recall that some part of the Kotlin community was working on a Kotlin-based port of a subset of java.time functionality to be used for Kotlin-based apps not running on a JVM.

I imagine that ported library works similar to code shown above.