mktime() timestamp calculation

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=4326

Reported by Erik Allemann erik@roxen.com

[RT#8983]

RTL having compatibility problems between 2.4 editserver and 4.5-rel2 frontends.

Code excerpt:

mapping mtime = localtime(time(1)) + ([ "min" : 0, "sec" : 0, "hour" : 0
]);
mtime->mday -= 2; // will cause a mtime->mday = -1;
int newtime = mktime(mtime);

write(ctime(newtime));

The code is supposed to calculate an timestamp (2 days) in the past. On pike 7.2 the resulting timestamp will be of February 27th, on pikes 7.4 and 7.6 the calculated timestamp will be March 2nd.