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

2
votes

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?

2
c++datetime

2 Answers

2
votes

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.

1
votes

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.