The Cox model is the generalization of the simple hazard ratio. It's about the most common test in survival statistics.

Everyone with 14 day data goes into calculating P(survive 14)
Everyone who has 28 data and was alive at 14 goes into calculating P(survive 28 given survive 14)
Everyone who has 42 day data and was alive at 28 goes into calculating P(survive 42 given survive 28)

Then P(survive 42) is the product of those 3 terms. You get one P-value.