a post by Oddysey over at her blog. She mentions that she has no idea what the date actually is in the Doom & Tea Party's game. By the in-game calendar it is, in fact, the 20th day of the Second Moon of Spring. But that's not terribly important.
Generally speaking, in your standard Old School game there are few reasons why players worry about dates and time. The first is logistics: do they have enough food, principally. And food is measured in days-worth of rations. So knowing how many days it will take to travel from point A to point B, or how many days travel away from civilization they are is what they're really worried about.
And for that, you don't really need any calendar at all. But there are other issues that might make knowing specific dates important. Some of these are cultural: festivals, legal proceedings, birthdays, stuff like that. Some are merely window-dressing; are the peasants in the fields harvesting, sowing, mending fences or what? Some are logistical; are the roads smooth and dry, muddy swamps, or under two feet of snow?
But a few issues can be of vital importance to adventurers. The two that repeatedly pop up in my campaigns are the times of sunset and sunrise, and the phases of the moon. And for those, you need something a touch more robust. But only a touch. You'll notice that the calendar for Doom & Tea Parties is ridiculously simple and terribly unrealistic. But it makes it very easy to know both the season and the current phase of the moon. No math or tables required.
As it turns out, one of my players chose Howls at the Moon as their Focus, so knowing when the moon is full is actually pretty important. He doesn't have to guess, I don't have to fiddle with tables or calculations, and life is good. This is the Zen of the Lazy DM. ;)