Question

为什么这句话会回归真实？我知道我正在将Date与DateTime变量进行比较，但我正在寻找更具技术性的解释。

DateTime dt = DateTime.newInstance(2012,04,30,0,0,0);
system.debug(dt > Date.valueOf('2012-04-30'));

另外，2012-04-30之前的DateTime值（对于dt变量）是否也会返回true？

Answer 1

Superfell 在评论中指出，这可能是由于时区转换以及午夜（00:00:00）被追加到日期进行比较。简短的故事：他是正确的，因为午夜被附加到日期进行比较。详情如下。

我需要看到这一点，以便完全理解它;所以，我写了一段代码。

for(Integer mo=4; mo<6; mo++) // test April (4) and May (5) only
    for(Integer d=(mo==4?29:0); d<=(mo==5?2:30); d++) // test between 2012-04-29 and 2012-05-30
        for(Integer h=0; h<12; h++)
            for(Integer m=0; m<60; m++)
            {
                // this simulates a CreatedDate field (note it is generated using the newInstanceGMT method and not simply newInstance)
                // DateTime fields in Salesforce are stored in GMT then converted to the current User's time-zone. 
                DateTime dt1 = DateTime.newInstanceGMT(2012,mo,d,h,m,0); 
                Date dt2 = Date.valueOf('2012-04-30');
                if(dt1 > dt2) { 
                    system.debug(LoggingLevel.INFO,'DateTime:'+dt1+' > Date:'+dt2); }
            }

显示生成的调试日志：

USER_DEBUG|[9]|INFO|DateTime:2012-04-30 00:01:00 > Date:2012-04-30 00:00:00
USER_DEBUG|[9]|INFO|DateTime:2012-04-30 00:02:00 > Date:2012-04-30 00:00:00
USER_DEBUG|[9]|INFO|DateTime:2012-04-30 00:03:00 > Date:2012-04-30 00:00:00
...
USER_DEBUG|[9]|INFO|DateTime:2012-05-02 11:57:00 > Date:2012-04-30 00:00:00
USER_DEBUG|[9]|INFO|DateTime:2012-05-02 11:58:00 > Date:2012-04-30 00:00:00
USER_DEBUG|[9]|INFO|DateTime:2012-05-02 11:59:00 > Date:2012-04-30 00:00:00

简化解决方案将与Date.valueOf('2012-05-01');进行比较。但是，可以使用DateTime类型上的newInstanceGMT方法与CreatedDate字段进行比较。使用DateTime方法，需要考虑午夜。

DateTime dt1 = DateTime.newInstanceGMT(2012,mo,d,h,m,0); // simulated CreatedDate field
Date dt2 = Date.valueOf('2012-05-01'); 
if(dt1 > dt2) { 
    system.debug(LoggingLevel.INFO,'DateTime:'+dt1+' > Date:'+dt2); }

OR

DateTime dt1 = DateTime.newInstanceGMT(2012,mo,d,h,m,0); // simulated CreatedDate field
DateTime dt2 = DateTime.newInstanceGMT(2012,04,30,12,59,59); 
if(dt1 > dt2) { 
    system.debug(LoggingLevel.INFO,'DateTime:'+dt1+' > Date:'+dt2); }

这两种方法都产生了预期的结果：

USER_DEBUG|[9]|INFO|DateTime:2012-05-01 00:00:00 > Date:2012-04-30 12:59:59
USER_DEBUG|[9]|INFO|DateTime:2012-05-01 00:01:00 > Date:2012-04-30 12:59:59
USER_DEBUG|[9]|INFO|DateTime:2012-05-01 00:02:00 > Date:2012-04-30 12:59:59
...
USER_DEBUG|[9]|INFO|DateTime:2012-05-02 11:57:00 > Date:2012-04-30 12:59:59
USER_DEBUG|[9]|INFO|DateTime:2012-05-02 11:58:00 > Date:2012-04-30 12:59:59
USER_DEBUG|[9]|INFO|DateTime:2012-05-02 11:59:00 > Date:2012-04-30 12:59:59

日期时间和日期比较

1 个答案: