Question

标题说明了一切。互联网上有很多例子如何将FILETIME转换为TDateTime但不是相反。

Answer 1

从DateTimeToSystemTime致电System.SysUtils。
从SystemTimeToFileTime致电Winapi.Windows。

如果您的原始日期时间值是当地时间，那么您需要在调用DateTimeToSystemTime之前将其转换为UTC。

Answer 2

您可以在ConvertDateTimeToFileTime中查看System.IOUtils的实现。它是TDirectory记录的私有成员。但是您可以将其用作参考，以便在一个功能中实现所需的所有功能。

Answer 3

function FileTimeToDateTime(AFileTime: FILETIME): TDateTime;
var
    li: ULARGE_INTEGER;
    systemTime: TSystemTime;
    localTime: TSystemTime;
const
    OA_ZERO_TICKS = UInt64(94353120000000000);
    TICKS_PER_DAY = UInt64(864000000000);
begin
    // Convert a FILETIME (which is UTC by definition), into a UTC TDateTime.

    // Copy FILETIME into LARGE_INTEGER to allow UInt64 access without alignment faults.
    li.LowPart := AFileTime.dwLowDateTime;
    li.HighPart := AFileTime.dwHighDateTime;
    Result := (Real(li.QuadPart) - OA_ZERO_TICKS) / TICKS_PER_DAY;
end;

function DateTimeToFileTime(ADateTimeUTC: TDateTime): FILETIME;
var
    li: ULARGE_INTEGER;
    systemTime: TSystemTime;
    localTime: TSystemTime;
const
    OA_ZERO_TICKS = UInt64(94353120000000000);
    TICKS_PER_DAY = UInt64(864000000000);
begin
    // Convert a UTC TDateTime into a FILETIME (which is UTC by definition).

    li.QuadPart := Round(ADateTimeUtc*TICKS_PER_DAY + OA_ZERO_TICKS);
    Result.dwLowDateTime := li.LowPart;
    Result.dwHighDateTime := li.HighPart;
end;

通过避免SYSTEMTIME，我们避免了losing precision in milliseconds.

根据定义，SYSTEMTIME的dwMilliseconds限制为1毫秒。

| FILETIME                       | SYSTEMTIME              |
|--------------------------------|-------------------------|
| 100ns resolution               | 1,000,000 ns resolution |
| GetSystemTimeAsFileTime        | GetSystemTime           |
| GetSystemTimePreciseAsFileTime | n/a                     |
|                   -- FileTimeToSystemTime -->            |
|                  <-- SystemTimeToFileTime --             |

您应避免使用SYSTEMTIME。 SYSTEMTIME的目的是为您将日期/时间解码为其组成部分：

年，月，日，时，分，秒，毫秒

如何将TDateTime转换为FILETIME？

3 个答案:

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