I have found this in functions.php:
// Add function mkrealdate for Birthday MOD
// the originate php "mktime()", does not work proberly on all OS, especially when going back in time
// before year 1970 (year 0), this function "mkrealtime()", has a mutch larger valid date range,
// from 1901 - 2099. it returns a "like" UNIX timestamp divided by 86400, so
// calculation from the originate php date and mktime is easy.
// mkrealdate, returns the number of day (with sign) from 1.1.1970.
// the originate php "mktime()", does not work proberly on all OS, especially when going back in time
// before year 1970 (year 0), this function "mkrealtime()", has a mutch larger valid date range,
// from 1901 - 2099. it returns a "like" UNIX timestamp divided by 86400, so
// calculation from the originate php date and mktime is easy.
// mkrealdate, returns the number of day (with sign) from 1.1.1970.
in italian: vien fuori questo errore dopo che imposto la data del forum dal server....