No you don't. You just have to assume that the Creator interfered at some point, in a way that doesn't fit the current laws of nature, and didn't go out of His way to tell us about it.
There are other explanations, but that is one of the quickest ways there.
Last Thursdayism and asserting random miracles unexplainable by science is at least a more honest position. Even though irrational, it at least accepts that the earth and the universe do scientifically appear to be older. But trying to claim the evidence shows the earth and the universe are 6000 years old is just plain nuts and inherently anti-science, except of course here on FR, where it is apparently given serious consideration and greater favor than long established and accepted science.
"Last Thursdayism" is about as accurate as calling the Strategic Defence Initiative "Star Wars". If you were merely using it as shorthand, seeing as it is a quick, vernacular term, OK; but otherwise it looks like something between a sneer and propaganda.
"Random miracles unexplainable by science" is almost a tautology. IF you start from the point of view that miracles are the result of deliberate actions by a sentinent supernatural being, they are not *random*. They only look that way because they cannot be deterministically predicted, or even probabilistically predicted on the basis of past physical events and the current state of the system under study. (And if they *are* miracles, then science won't be able to explain them, by definition. Why you feel that means the word must be used as a perjorative is another matter...)