You could use the System Event "Compare Time", two constants (START_TIME and END_TIME) and a number global variable as state (increaseDifficulty).
Set increaseDifficulty to 0.
Add a Compare Time event to check current time against START_TIME. If it is equal to START_TIME, you would set increaseDifficulty to 1.
Add a Compare Time event to check current time against END_TIME. If it is equal to END_TIME, you would set increaseDifficulty back to 0.
Also, you must set actions that increase game elements speed acording to the rule you want to be taken Every X Seconds if increaseDifficulty is 1.