Actualy, you can use item abilities for that.
Group the units you want to increase their hit points, and add the 'item life bonus' to it.
When your timer is done, you just remove the ability.
Otherwise, you can use the 'Item Permanent Life Gain'.
I'm not sure, but I think if you add negative values to it, you may also decrease the hit points.
Upgrades is a fine alternative too if you want to make it easy.
Yet remember, if the player is a human, he will hear the sounds of 'update completed'!