1569
Sea Level
(imgs.xkcd.com)
Alt text: They're up there with coral islands, lightning, and caterpillars turning into butterflies.
13*28=364 so even 13 months and 28 days doesn't work.
If we had 28 days in a month then the week needs to be something other than 7 days. Three out of four times February / March fucks me over by having the same weekday/ day of the month.
There is a calendar that uses 28, 35, 28 day months each quarter for 364 days, with the last quarter having an extra week (or having an intercalary week so they can pretend quarters will be all equal) in leap years