It is true that in simple sentences most adverbs are placed directly after the verb most of the time. There are exceptions - including many adverbs of time. Not all the variation in position of adverbs is covered in the lessons - some just have to be learnt by noting usage in writing and in speech. The attached covers adverbs of time in compound tenses, but the placement in your example sentence is similarly determined.

Position of French Adverbs - with compound tenses

Hi Olaedo,

I'm thinking that "plus tôt" means "earlier" in the sense that the alarm clock will be set to an earlier hour. I don't think that it would be logical to interpret "I will put earlier ..."

Hope this helps.

Jim

Bonjour Oleado. I believe the answer to your question is that "plut tot" in this case is an adjectival phrase describing the alarm clock rather than an adverbial phrase describing the action of setting the alarm clock. That is, Lisa wants to set the clock to an earlier time. If "plus tot" were to immediately follow "mettrai" than it would mean that Lisa would herself, at an earlier time, be performing the action of setting her alarm clock, (not necessarily the time to which the clock has been set). 

Hope this helps.