Without sounding too pessimistic, there likely can never be a good solution to this problem, because time in UO is fundamentally broken in nearly every possible way. For one, it's impossible to really know what the date actually is. Stratics has a calculator but this is said to create an incorrect time. The structure of time was also arbitrarily changed at some point, yet without any in-game means to determine the current date, any supposed rules are merely hypothetical. In other words, there are no rules.
Just to make matters worse, even the in-game clock is out of whack. A few months ago I tried to create a date calculator and time converter based on current earth time, and I spent some time observing the in-game clock to mark down certain time points, with the assumption that it would be the same Britannian time at the same earth time, every day. I finished the converter and watched it display Britannian time. After awhile, I noticed that the time no longer matched what the in-game clocks would say. As it turns out, the in-game clock was running slow. You see, my converter would generate the current Britannian time based on the current Earth time. I have reason to believe that the in-game clock doesn't convert time at all and doesn't actually run at the 5 seconds to 1 Britannian minute rule that it is supposed to use (instead, running faster than it ought to). Moral of the story is that UO time is fundamentally broken.
All that being said, it ought to be a 1:1 ratio at least in terms of the game's coding. The reason is, while UO lore may dictate that Britannia time is faster, my character does not (or rather, should not) experience time at an accelerated rate. In other words time runs faster but my character does not (because I cannot, since I cannot perceive time at the rate that my character supposedly perceives time). Therefore, the way I use time when I play is that a Britannian day runs alongside an Earth day, so if I see someone in the evening I will say Good Evening. If I see someone in the morning I'll say Good Morning. If it's winter in real life, I'll say it's winter in game and act accordingly.
As for the passing of years, how is that supposed to work? Am I supposed to be aging my character by 5 years every real year? What is the life expectancy of a human? Do we have 200 year old humans walking around, because that doesn't seem right to me at all. When you consider that a large portion of roleplayers like to play characters that are between ages 18-25, it seems like one would run into a lot of conflicts between technical accuracy and personal preferences.
My view then is to disregard the passing of years for the most part, and treat Britannia like a world set at a single time. After all, there is no reliable means to measure time, no hard-coded in-game rules, and no one way between RPers to observe it. This is just my general approach, which I view as the simplest and easiest way to look at time.