时间与相关类型(2) TDate、TTime、TTimeStamp

//相关函数:
SysUtils.DateTimeToTimeStamp();
SysUtils.TimeStampToDateTime();

SysUtils.TimeStampToMSecs();
SysUtils.MSecsToTimeStamp();

Windows.GetTickCount;
Windows.GetCurrentTime; {GetCurrentTime 和 GetTickCount 一样, 是调用了 GetTickCount.}

TDate、TTime 与 TDateTime 的尺寸是一样的, 互相转换也不会损失数据; 存在的意义也只是便于理解.

比较有意思的是 TTimeStamp 结构:

TTimeStamp = record
  Time: Integer;      { "毫秒" 表示的时间, 自: 0:0:0:0 }
  Date: Integer;      { "天" 表示的日期, 自: 1/1/0001 }
end;


{TDateTime 与 TTimeStamp 互相转换的例子}
var
  T: TDateTime;
  TS: TTimeStamp;
begin
  T := EncodeDateTime(2009, 5, 22, 11, 22, 33, 999);
  TS := DateTimeToTimeStamp(T);

  ShowMessage(IntToStr(TS.Time)); //40953999; 当日已逝去了 40953999 毫秒
  ShowMessage(IntToStr(TS.Date)); //733549;   自 0001-1-1 的第 733549 天

  T := TimeStampToDateTime(TS);
  ShowMessage(FormatDateTime('yyyy-m-d h:m:s:z', T)); //2009-5-22 11:22:33:999
end;

假如我们要想知道自 0001-1 0:0:0:0 到 2009-5-22 11:22:33:999 总的毫秒数, 根据上面的结果不难算出:
733549*24*60*60*1000 + 40953999 = 63378674553999.

不过这有现成的函数: TimeStampToMSecs、MSecsToTimeStamp.

---

{TimeStampToMSecs、MSecsToTimeStamp 示例:} 
var
  T: TDateTime;
  TS: TTimeStamp;
  ms: Comp; {这是 TimeStampToMSecs 返回的类型}
begin
  T := EncodeDateTime(2009, 5, 22, 11, 22, 33, 999);
  TS := DateTimeToTimeStamp(T);

  ms := TimeStampToMSecs(TS);
  ShowMessage(FloatToStr(ms)); //63378674553999

  TS := MSecsToTimeStamp(ms);
  T := TimeStampToDateTime(TS);
  ShowMessage(FormatDateTime('yyyy-m-d h:m:s:z', T)); //2009-5-22 11:22:33:999
end;

另外: GetTickCount 函数可获取从开机时间到当前时间的毫秒数, 结合上面, 可以算出开机时间:

---

{获取开机时间的例子}
var
  T1: Int64;
  T2,T3: Comp;
  T4: TDateTime;
begin
  T1 := GetTickCount;                               {从开机到现在的毫秒数}
  T2 := TimeStampToMSecs(DateTimeToTimeStamp(Now)); {从 0001-1-1 到当前时间的毫秒数}
  T3 := T2 - T1;                                    {从 0001-1-1 到开机时刻的毫秒数}
  T4 := TimeStampToDateTime(MSecsToTimeStamp(T3));  {从 0001-1-1 到开机时刻的时间}
  ShowMessage(DateTimeToStr(T4));                   {显示开机时间}
end;