Decreased Magic Casting Time

From FlyFF WIki

Note: Today, DMMT is typically called DCT, or Decreased Casting Time. DMMT is the old notation.

Or DMMT
These terms for naming the the bonus that affects the time you take to cast skills can be quite confusing.

Hypothetical amounts used for explanation

To decrease the time it takes to cast any numbers that are positive will reduce the amount of time needed. Any negative reductions would add to the time.

Imagine your casting time takes 5 seconds to cast.

+20% decrease: would be equal to removing one second

20% of 5 = 1

so
+20% casting time is now 4 sec

-20% decrease: would be a double negative thus adding to the time it takes to cast. So you would add a second.

20% of 5 = 1

so
-20% casting time is now 6 sec