How to compare string date with time_t in c++?

Question

I have string below.

std::string _valid = "2014-03-28 16:45:59";

int y1, M1, d1, h1, m1, s1;
sscanf_s(_valid.c_str(), "%d-%d-%d %d:%d:%d", &y1, &M1, &d1, &h1, &m1, &s1);

Now, I can get year, month, day and so on...

Now, I have to determine whether that time is earlier than current datetime.

time_t now  = time(NULL);

But, time_t always returns me the time in unix timestamp.

How can I compare the date?

Answer 1

Set to struct tm , then generate time_t by function mktime . You can compare the value of two time_t directly.

You can also use SYSTEMTIME and FILETIME from windows.h. Set to SYSTEMTIME then generate FILETIME by function SystemTimeToFileTime . Compare two FILETIME by function CompareFileTime . To compute the distance between two FILETIME, I'm writing the follow ugly code:

__int64 duration = ((*(__int64 *)&ft2) - (*(__int64 *)&ft1));

Note that SYSTEMTIME has higher resolution than struct tm.

Answer 2

I think you are looking for the mktime function. The details are here http://linux.die.net/man/3/mktime , but in a nutshell...

#include <time.h>
time_t mktime(struct tm *tm);

where the struct tm is defined in time.h as...

struct tm
{
  int tm_sec;           /* Seconds. [0-60] (1 leap second) */
  int tm_min;           /* Minutes. [0-59] */
  int tm_hour;          /* Hours.   [0-23] */
  int tm_mday;          /* Day.     [1-31] */
  int tm_mon;           /* Month.   [0-11] */
  int tm_year;          /* Year - 1900.  */
  int tm_wday;          /* Day of week. [0-6] */
  int tm_yday;          /* Days in year.[0-365] */
  int tm_isdst;         /* DST.     [-1/0/1]*/

#ifdef  __USE_BSD
  long int tm_gmtoff;       /* Seconds east of UTC.  */
  __const char *tm_zone;    /* Timezone abbreviation.  */
#else
  long int __tm_gmtoff;     /* Seconds east of UTC.  */
  __const char *__tm_zone;  /* Timezone abbreviation.  */
#endif
};

Hope this helps.