Am I allowed to use a commercial warmer on Shabbos with a timer, so I can put things in before Shabbos and the timer turns off itself on Shabbos making me be able to take whatever is in there out after it turns off?

 

Yes.

However, the food must be at least half cooked before Shabbos, see halacha2go.com #753.

ראה שאלה #3506:

Is an oven with Cook-time or Stop-time Mode acceptable for Shabbos use?

 

 

#5506