Time convert on c++

Background

UTC: Coordinated Universal Time.

GMT: Greenwich Mean Time.

FILETIME:

LOCALTIME:

SYSTEMTIME:

STRUCTURE/DEFINITION

For windows

SYSTEMTIME structure. The time is either in coordinated universal time (UTC) or local time, depending on the function that is being called.

typedef struct _SYSTEMTIME { 
  WORD wYear; 
  WORD wMonth;          // based on 1(Jan)
  WORD wDayOfWeek;      // 0 for sunday, 1 for monday, … 6 for saturday
  WORD wDay;            // based on 1
  WORD wHour;           // from 0 to 23
  WORD wMinute;         // from 0 to 59
  WORD wSecond;         // from 0 to 59
  WORD wMilliseconds;   // from 0 to 999
} SYSTEMTIME;

FILETIME structure. This value represents the number of 100-nanosecond units since the beginning of January 1, 1601.

typedef struct _FILETIME { 
  DWORD dwLowDateTime; 
  DWORD dwHighDateTime; 
} FILETIME, FAR *LPFILETIME; 

ULARGE_INTEGER Union

typedef union _ULARGE_INTEGER {  
    struct {    
        DWORD LowPart;    
        DWORD HighPart;  
    };  
    struct {    
        DWORD LowPart;    
        DWORD HighPart;  
    } u;  
    ULONGLONG QuadPart;
} ULARGE_INTEGER,  
 *PULARGE_INTEGER;

Note: As the structure shown above.

FILETIME –> ULARGE_INTEGER: Do not cast a pointer to a FILETIME structure to either a ULARGE_INTEGER* or __int64* value because it can cause alignment faults on 64-bit Windows. You should copy the low- and high-order parts of the file time to a ULARGE_INTEGER structure, perform 64-bit arithmetic on the QuadPart member.

ULARGE_INTEGER –> FILETIME: Cast pointer to ULARGE_INTEGER to FILETIME or copy the LowPart and HighPart members into the FILETIME structure.

time_t (__int64 or long integer)

struct tm {
  int  tm_sec;        // from 0 to 59
  int  tm_min;        // from 0 to 59
  int  tm_hour;       // from 0 to 23
  int  tm_mday;       // from 1 to 31
  int  tm_mon;        // from 0 to 11
  int  tm_year;       // Indicates the number of years since 1900. 
  int  tm_wday;       // 0 for sunday, 6 for saturday
  int  tm_yday;       // from 0 to 365 
  int  tm_isdst;
};

Retrieve Function

From total seconds (clocks) to tm structure for local time, lval is the variable for total seconds.

time_t t(lval);
struct tm  * pltm = localtime((const time_t*)&t);

From total seconds (clocks) to tm structure for gmt time, lval is the variable for total seconds.

time_t t(lval);
struct tm * pgtm = gmtime((const time_t*)&t);
cout<<asctime((const tm*)pgtm);

From tm structure to calendar value

time_t _t = mktime(pltm);