确定日期开始的时间 [英] Determining the time at which a date starts

问题描述

说我想创建一个每日策划师，我想把这一天分成15分钟。

容易，对吧？从午夜开始，...错误！在美国/圣保罗，由于夏令时更改，每年有一天从01:00开始。

给定时区和日期，如何找到时代一天开始的时间？

我的第一个想法是使用以下内容，但假设每一天都有23:59。这可能不如假设每一天都有一个午夜。

  perl -MDateTime -E'
说
 DateTime-> new（year => 2013，month => 10，day => 20）
  - > subtract（days => 1）
  - > set（hour => 23，minute => 59）
  - > set_time_zone（America / Sao_Paulo）
  - > add（minutes => 1）
  - > ; strftime（％H：％M）; 
'
 01:00 
  

有更强大或更直接的替代方法？

解决方案

[此功能现已从 DateTimeX ::开始 ]

这里是一个仅使用DT公开的解决方案方法：

  sub day_start {
 my（$ y，$ m，$ d，$ tz）= @_ ; 
 
 $ tz = DateTime :: TimeZone-> new（name => $ tz）
 if！ref（$ tz）; 
 
我的$ dt = DateTime-> new（year => $ y，month => $ m，day => $ d）; 
我的$ target_day =（$ dt-> utc_rd_values）[0]; 
我的$ min_epoch = int（$ dt-> epoch（）/ 60） -  24 * 60; 
 my $ max_epoch = int（$ dt-> epoch（）/ 60）+ 24 * 60; 
 while（$ max_epoch> $ min_epoch）{
我的$ epoch =（$ min_epoch + $ max_epoch）>> 1; 
 $ dt = DateTime-> from_epoch（epoch => $ epoch * 60，time_zone => $ tz）; 
 if（（$ dt-> local_rd_values）[0]< $ target_day）{
 $ min_epoch = $ epoch; 
} else {
 $ max_epoch = $ epoch; 
} 
} 
 
返回DateTime-> from_epoch（epoch => $ max_epoch * 60，time_zone => $ tz）; 
} 
  

由于大多数日期都有午夜，所以应该在顶部添加一个支票在不需要时绕过搜索。

假设：

• 没有dt可以添加时间以获得具有较早日期的dt。


• 在时区开始之前，日期开始时间不超过24 * 60 * 60秒。


• 在时区开始后，24小时不得超过24 * 60 * 60秒。


• 只发生在零秒的时间。 （优化）

测试：

  {
＃没有午夜。 
 my $ tz = DateTime :: TimeZone-> new（name =>'America / Sao_Paulo'）; 
我的$ dt = day_start（2013，10，20，$ tz）; 
 print（$ dt-> epoch，，$ dt-> iso8601，\\\
）; ＃1382238000 2013-10-20T01：00：00 
 $ dt-> subtract（seconds => 1）; 
 print（$ dt-> epoch，，$ dt-> iso8601，\\\
）; ＃1382237999 2013-10-19T23：59：59 
} 
 
 {
＃两个半夜。 
 my $ tz = DateTime :: TimeZone-> new（name =>'America / Havana'）; 
我的$ dt = day_start（2013，11，3，$ tz）; 
 print（$ dt-> epoch，，$ dt-> iso8601，\\\
）; ＃1383451200 2013-11-03T00：00：00 
 $ dt-> subtract（seconds => 1）; 
 print（$ dt-> epoch，，$ dt-> iso8601，\\\
）; ＃1383451199 2013-11-02T23：59：59 
} 
  

Say I want to create a daily planner, and I want to divide the day into 15 minute chunks.

Easy, right? Just start at midnight, and... Wrong! In America/Sao_Paulo, one day each year starts at 01:00 because of Daylight Saving Time changes.

Given a time zone and a date, how does one find the epoch time at which the day starts?

My first thought was to use the following, but it assumes each day has a 23:59. That's probably no better of an assumption than assuming each day has a midnight.

perl -MDateTime -E'
   say
      DateTime->new( year => 2013, month => 10, day => 20 )
      ->subtract( days => 1 )
      ->set( hour => 23, minute => 59 )
      ->set_time_zone("America/Sao_Paulo")
      ->add( minutes => 1 )
      ->strftime("%H:%M");
'
01:00

Is there a more robust or more direct alternative?

解决方案

[This functionality is now available from DateTimeX::Start]

Here's a solution using only DT's public methods:

sub day_start {
   my ($y, $m, $d, $tz) = @_;

   $tz = DateTime::TimeZone->new( name => $tz )
      if !ref($tz);

   my $dt = DateTime->new( year => $y, month => $m, day => $d );
   my $target_day = ( $dt->utc_rd_values )[0];
   my $min_epoch = int($dt->epoch()/60) - 24*60;
   my $max_epoch = int($dt->epoch()/60) + 24*60;
   while ($max_epoch > $min_epoch) {
      my $epoch = ( $min_epoch + $max_epoch ) >> 1;
      $dt = DateTime->from_epoch( epoch => $epoch*60, time_zone => $tz );
      if (( $dt->local_rd_values )[0] < $target_day) {
         $min_epoch = $epoch;
      } else {
         $max_epoch = $epoch;
      }
   }

   return DateTime->from_epoch(epoch => $max_epoch*60, time_zone => $tz);
}

Since most dates do have a midnight, a check should be added at the top to bypass the search when it's not needed.

Assumptions:

• There is no dt to which one can add time to obtain a dt with an earlier date.
• In no time zone does a date starts more than 24*60*60 seconds before the date starts in UTC.
• In no time zone does a date starts more than 24*60*60 seconds after the date starts in UTC.
• Jumps in time zones only occur on times with zero seconds. (Optimization)

Test:

{
    # No midnight.
    my $tz = DateTime::TimeZone->new( name => 'America/Sao_Paulo' );
    my $dt = day_start(2013, 10, 20, $tz);
    print($dt->epoch, " ", $dt->iso8601, "\n");  # 1382238000 2013-10-20T01:00:00
    $dt->subtract( seconds => 1 );
    print($dt->epoch, " ", $dt->iso8601, "\n");  # 1382237999 2013-10-19T23:59:59
}

{
    # Two midnights.
    my $tz = DateTime::TimeZone->new( name => 'America/Havana' );
    my $dt = day_start(2013, 11, 3, $tz);
    print($dt->epoch, " ", $dt->iso8601, "\n");  # 1383451200 2013-11-03T00:00:00
    $dt->subtract( seconds => 1 );
    print($dt->epoch, " ", $dt->iso8601, "\n");  # 1383451199 2013-11-02T23:59:59
}

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