Dog heartworm question?

my dog started taking her heartworm on the 26th a month ago, and i set up an alarm for this... the vet said i had to be precise in giving her the chewable on the 26th every month. This month my alarm even though set (i checked it) didnt go off and i missed the 26th and gave her the medication on the 27th instead, is this bad or now do i have to go get her tested again?