This could happen when it was called for a time other than the present, because it set tm_isdst based on the current value of the DST flag in BRAM, which is valid only for the current time.
Now it works as follows: If localtime() is passed the time_t value that was produced by the most recent call to time(), it gives the DST setting in effect at the time of that call. Otherwise, it sets tm_isdst to -1, indicating that the correct value is unknown.
This is about the best we can do without having a full timezone library/database. It should give the right tm_isdst value in probably the most common case, and avoids giving an incorrect value in any case (assuming the system's DST flag is right).
This fixes#18.
