Of course, if you do write your own code you will be
spending time to re-implement something that someone
has already written. If the overhead of calling a
"slow" module like Date::Manip is too much for you, then
Perl is probably too slow and inefficient for you; you should
use a compiled language like C.
Also Date::Manip (and all other
perl modules) has been used
by thousands of perl programmers, so the code
is well tested and proven to be reliable, stable
and relatively bug-free.