c++ - SYSTEMTIME 上的 30 天差异

Question

我想问一下是否有人有一些好的算法(或方法)来检查两个 SYSTEMTIME 变量是否有 30 天或更长时间的差异？

谢谢

Answer 1

作为MSDN page在 SYSTEMTIME 上说，

It is not recommended that you add and subtract values from the SYSTEMTIME structure to obtain relative times. Instead, you should

• Convert the SYSTEMTIME structure to a FILETIME structure.
• Copy the resulting FILETIME structure to a ULARGE_INTEGER structure.
• Use normal 64-bit arithmetic on the ULARGE_INTEGER value.

SYSTEMTIME st1, st2;
/* ... */
FILETIME ft1, ft2;
ULARGE_INTEGER t1, t2;
ULONGLONG diff;
SystemTimeToFileTime(&st1, &ft1);
SystemTimeToFileTime(&st2, &ft2);
memcpy(&t1, &ft1, sizeof(t1));
memcpy(&t2, &ft2, sizeof(t1));
diff = (t1.QuadPart<t2.QuadPart)?(t2.QuadPart-t1.QuadPart):(t1.QuadPart-t2.QuadPart);
if(diff > (30*24*60*60)*(ULONGLONG)10000000)
{
    ...
}

(为简洁起见省略了对调用 SystemTimeToFileTime 的错误处理)

关于(30*24*60*60)*(ULONGLONG)10000000:30*24*60*60是30天的秒数； 10000000 是 FILETIME 的个数一秒钟“滴答”(每个 FILETIME 滴答是 100 ns=10^2*10^-9 s=10^-7 s)。转换为乘法中的操作数之一是为了避免溢出常规 int(在适合 int 范围的整数文字之间执行运算时的默认设置)。