Hi, boost::gregorian::to_tm() initializes some fields of struct tm, but not all. Implementations keep there additional fields. Besides the fields defined by POSIX libc2.9 has two additional fields in struct tm: struct tm { ... #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 }; There are two additional fields tm_gmtoff and tm_zone. boost::gregorian::to_tm() does not initialize tm_zone and tm_gmtoff. There are functions as well which use these additional fields. E.g. this is a piece of code from the implementation of strftime() from libc2.9: static size_t __strftime_internal (s, maxsize, format, tp, tzset_called ut_argument ... { ... zone = (const char *) tp->tm_zone; #endif #if HAVE_TZNAME if (ut) { if (! (zone && *zone)) zone = "GMT"; } ... } This function dereferences tp->tm_zone. I have not created a ticker since this could be an already known issue. BR, Dmitry