[Gretl-devel] Re: questions about Artur's calendar_utils package

On Thu, 30 Sep 2021, Riccardo (Jack) Lucchetti wrote: Here's the issue. strptime() gives you the time in the form of a "struct tm", and strftime() wants a struct tm as input. But there's nothing in hansl corresponding to struct tm, so we convert the struct we get from strptime to calendar representation as a "time_t" (basically, an integer) using the C-library function mktime(). Then we convert back from time_t to struct tm if and when strftime is called. The trouble is that this round-trip goes wrong for an underspecified time such as plain "1984". Given that input, strptime leaves the "mday" (day-of-month) member of the tm struct at 0, but that's not a valid value, mday has to be in the range 1 to 31. So mktime looks at the invalid 0th of January 1984 and "corrects" it, as best it can, to the 31st of December 1983. The fix is then to initialize mday to 1 before calling strptime. If the input specifies a day of the month the "1" will get overwritten; if it fails to supply a day of the month we default to the first. That fix is now in git. (This is kinda confusing: 0 is a valid value for all members of struct tm -- for instance, internally January is month 0 -- except for the day of the month.) Allin