construct date in a different locale than current - c++

Question

I have a requirement, where a datetime, locale and daylight savings is input as fixed length string, such as YYMMDDHHMMCY, where the legend is provided below.
YY(year)
MM(month)
DD(day)
HH(hour)
MM(minute)
Timezone (C for central, P for Pacific, M for mountain, E for eastern)
Daylight savings (Y if daylight savings is in effect, other wise N)
What is needed is the ability to construct time in a specified timezone and then convert it to local timezone in C/C++. We dont use Boost, are there existing functions which would allow for the requirements. I am aware of strptime, which after some massaging the data, i can use it, but i am wondering if there are functions as described above which would allow me to construct a struct in a specified locale.

Answer 1

Use sscanf() or strptime() to extract most of the fields. The timezone character and DST character will need to be decoded on their own. Since you are using only a 2 digit year, your need to define your range. Example below uses 1970-2069. Use the extracted timezone character to form the usual timezone name. Before calling mktime(), set TZ to the timezone name. Then, with a time_t in hand, convert to your local time.
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
extern time_t mktime_TZ(struct tm *tm, const char *tz);
extern time_t DecodeTimeString_time_t(const char *time_string);
void DecodeTimeString_Local(const char *time_string, struct tm *local) {
// Various error handling not shown
time_t t;
t = DecodeTimeString_time_t(time_string);
*local = *localtime(&t);
}
time_t DecodeTimeString_time_t(const char *time_string /* YYMMDDHHMMCY */) {
struct tm tm;
char Zone, DST;
int result = sscanf(time_string, "%2d%2d%2d%2d%2d%[CEMP]%[NY]",
&tm.tm_year, &tm.tm_mon, &tm.tm_mday, &tm.tm_hour, &tm.tm_min, &Zone, &DST);
if (result != 7) {
; // handle error
}
// Your need to decide how to handle 2 digits years
// Assume 70-99 is 1970-1999 and 0 to 69 is 2000-2069
if (tm.tm_year < 70) tm.tm_year += 2000-1900;
tm.tm_mon--; // Assume DateString used "01" for January, etc.
tm.tm_sec = 0;
tm.tm_isdst = Zone == 'Y';
const char *TZ;
switch (Zone) {
case 'P': TZ = "PST8PDT"; break; // Pacific
case 'M': TZ = "MST7MDT"; break; // mountain
case 'C': TZ = "CST6CDT"; break; // central
case 'E': TZ = "EST5EDT"; break; // eastern
}
time_t t = mktime_TZ(&tm, TZ);
return t;
}
// Form time_t from struct tm given a TZ
time_t mktime_TZ(struct tm *tm, const char *tz) {
time_t t;
const char *old_tz = getenv("TZ");
if (setenv("TZ", tz, 1 /* overwrite */)) {
return -1; // handle error
}
tzset();
t = mktime(tm);
if (old_tz) {
if (setenv("TZ", old_tz, 1 /* overwrite */)) {
return -1; // handle error
}
}
else {
if (unsetenv("TZ")) {
return -1; // handle error
}
}
tzset();
return t;
}