Bug in UTC -> localtime conversion after 2038

[Udo de Boer]

Hi,

UTC time to local timezone conversion did not work for years after 2038. 
Daylight saving in the timezone was not recognized. Tested on 32bit 
xtensa architecture with GCC 8.4 (esp32) and with 64bit time_t enabled.

The struct tm is filled correct. So year, day etc are all filled with 
the correct values. But the tm.tm_isdst is 0 and no daylight saving is 
applied.

It was caused by the following calculation in time/tzcalc_limits.c at 
line 68

        /* store the change-over time in GMT form by adding offset */
        tz->__tzrule[i].change = days * SECSPERDAY +
        tz->__tzrule[i].s + tz->__tzrule[i].offset;

Here tz->__tzrule[i].change is a time_t (64bit). For some reason the 
compiler does the calculation in 32 bit. All variables used in the 
calculation are 32bit.

I solved this locally by changing the lines to.

        /* store the change-over time in GMT form by adding offset */
        tz->__tzrule[i].change = (time_t)days * SECSPERDAY +
        tz->__tzrule[i].s + tz->__tzrule[i].offset;

Adding the cast time_t should not cause a problem when time_t is still 
32bit.

I don't know if this is correct solution. I also only needed this 
function. So more bugs like this could be hidden away in the time code.