do val_int() on their arguments before starting the computation. 
Similar fixes are need for +-* and probably several other but I want
to make sure Monty is fine with my fix approach before changing a lot
of code. 

Amazingly,
this bug is not as critical as you would expect since very few functions do val_int()
on their arguments ( from_unixtime(), sec_to_time()), and those not
very frequently perform a computation on their floating point arguments.
which is probably why no one has yet reported this bug. Another 
possibility is that the result is usually wrong by no more than 5%,
which makes it hard to catch it. I found it when trying to compute mile
splits for 30:47 10K - it told me 5:07, and I knew it was wrong because
5:00 mile gives you 31:08. However, if I had not run as many 10K races,
I would have easily believed that 30:47 10K is a 5:07 mile pace and 
would not have noticed the bug.


mysql-test/r/func_time.result:
  another test for sec_to_time that exposes a long outstanding bug
mysql-test/t/func_time.test:
  another test for sec_to_time that exposes a long outstanding bug
sql/item_func.cc:
  fixed bug in Item_func_div::val_int()