For doing this (changing ms by given amount) I recommend triggering it, since normal item ability - 'boots of speed' does not stack. Use real variables for storing current movement spell of unit, and second one for storing amount of ms reduction.
Lets say C_real - current; R_real - reduction.
Then use action Use - Set Current memvement speed of your_unit to C_real - R_real.
Use proper indexing, (dont use waits) and after given amount of time add the value R_real again to your_unit.
However, it would be much simpler if you'd use %ms abilityies, just choose one of many agilities that change ms by % (eg commanding aura) and time it.
Look my signature for timed A/S system, its made for timing abilities.
If I'm mistaken about abilities about ms (not %), I'm sorry, don't use such ofen.