关于各种Timestamps的区别和比较：

UNIX timestamp: seconds since midnight, January 1, 1970 UTC. It is a 32-bit number.
Win32 FILETIME values count 100-nanosecond intervals since January 1, 1600 UTC. It is a 64-bit number.
CLR System.DateTime values count 100-nanosecond intervals since January 1, 1 UTC. It is a 64-bit number. These aren’t used much yet.
The DOS date/time format is a bitmask (YYYYYYYM MMMDDDDD hhhhhmmm mmmsssss) The year is stored as an offset from 1980. Seconds are stored in two-second increments. These values are recorded in local time.
The OLE automation date (DATE) format is a floating point value, counting days since midnight 30 December 1899. Hours and minutes are represented as fractional days.
The C runtime time_t value is the same as a UNIX timestamp, __time64_t is same except using 64-bit number

* time_t: 19:14:07, January 18, 2038, UTC.
* __time64_t: 23:59:59, December 31, 3000, UTC.

struct tm {
int tm_sec; /* seconds after the minute – [0,59] */
int tm_min; /* minutes after the hour – [0,59] */
int tm_hour; /* hours since midnight – [0,23] */
int tm_mday; /* day of the month – [1,31] */
int tm_mon; /* months since January – [0,11] */
int tm_year; /* years since 1900 */
int tm_wday; /* days since Sunday – [0,6] */
int tm_yday; /* days since January 1 – [0,365] */
int tm_isdst; /* daylight savings time flag */
};

typedef struct _SYSTEMTIME {
WORD wYear; /* no offset */
WORD wMonth; /* January is 1 */
WORD wDayOfWeek; /* Sunday is 0, Monday is 1, and so on.*/
WORD wDay; /* 1-31 */
WORD wHour;
WORD wMinute;
WORD wSecond;
WORD wMilliseconds;
} SYSTEMTIME, *PSYSTEMTIME, *LPSYSTEMTIME;

CTime 构造函数可以接受：

__time64_t(time_t),
{DosDate, DosTime},
SYSTEMTIME,
FILETIME,
{nYear: 1970-3000, nMonth: 1-12, nDay: 1-31, nHour, nMin, nSec}.
struct tm* GetGmtTm(struct tm* ptm = NULL) const;
struct tm* GetLocalTm(struct tm* ptm = NULL) const;
BOOL GetAsSystemTime(SYSTEMTIME& timeDest) const;
time_t GetTime() const;
int GetYear() const;
int GetMonth() const; // month of year (1 = Jan)
int GetDay() const; // day of month
int GetHour() const;
int GetMinute() const;
int GetSecond() const;
int GetDayOfWeek() const; // 1=Sun, 2=Mon, …, 7=Sat

COleDateTime 构造函数可以接收：

VARIANT,
DATE,
time_t,
SYSTEMTIME,
FILETIME,
{nYear: 100-9999, month: 0-12(from 1), day: 0-31 (from 1), hour: 0-23, minute: 0-59, second 0-59}
BOOL GetAsSystemTime(SYSTEMTIME& sysTime) const;
int GetYear() const;
int GetMonth() const; // month of year (1 = Jan)
int GetDay() const; // day of month (0-31)
int GetHour() const; // hour in day (0-23)
int GetMinute() const; // minute in hour (0-59)
int GetSecond() const; // second in minute (0-59)
int GetDayOfWeek() const; // 1=Sun, 2=Mon, …, 7=Sat
int GetDayOfYear() const; // days since start of year, Jan 1 = 1

http://www.codeproject.com/datetime/datetimedisc.asp

http://blogs.msdn.com/oldnewthing/archive/2003/09/05/54806.aspx