I had a friend carrying a Swedish passport do exactly that. he took a day trip and visited Tijuana from San Diego. In his case it did NOT reset the length of his current US stay. CBP treated it more or less as if he had never left the US. He was not trying to reset the length of his stay and was only in the US for a couple of weeks anyway so it was a non issue to him but I'm not sure he could have achieved a different result if he were trying to.