Time travel thing - a small preview

Forthcoming Cruises: Triassic-Cretaceous TS Corinthic, Captain Richard Thorne RNVR, White Star Line: Departs for a four-week cruise to the Triassic-Cretaceous oceans, calling in at London (1588 AD), Vinland (1000 AD), Herculaneum (75 AD; day trip to Pompeii available), Africa (500,000 BC; hunting trips available), Triassic-Cretaceous (circa 150 Million BC). Launched in 1902, the Corinthic is a luxurious new liner of 12,251 tons capable of 14 knots and equipped with the latest Ferguson Chronatron® time displacer and a Babbage-Williamson navigational engine. Excluding stops en-route and any delays to manoeuvre the journey to the Triassic-Cretaceous seas will take approximately a week, with a week spent sight-seeing and hunting in this period before the return journey. Currently all first-class accommodation is reserved, but second and third-class cabins are still available and may be booked via Thomas Cook & Sons and other reputable agents. Corinthic will depart from pier 7, Southampton, at 2.00 PM on Tuesday 11th May.

Okay - showing my crappy maths here. What I need here is a SIMPLE mathematical formula that'll relate the distance travelled in time to the distance at sea, possibly an exponential curve or close to it. The way I want it to work is that you start off slowly but accelerate. A few months in the first few miles, a few years in the next few, and so forth, say 200 million years in a week, and the beginning of the universe in a month or two (assuming you could keep sailing). I can simply give figures that suit me, but it'd be better if I could give a formula that can be used without too much messing around. Any suggestions?