The answer may sound a bit tongue-in-cheek, but there is a mutation involved. Welsh has two so-called “positive markers” fe and mi, and their only purpose is to mark that the following sentence is a positive statement. These markers cause soft mutation. So the usual explanation is that the marker itself has been dropped from speech, but the mutation is kept.
So (Mi) ges i amser da. and Ces i amser da. are both completely fine and interchangeable.