The calendar of Harptos breaks the year up into twelve months of thirty days, along with five (or six on the leap-year) inter-month days. The months are broken up into three "tendays" or "rides".
Days are broken up into 24 hours, twelve daylight hours, twelve night hours. Hours are measured from sunup to sundown, so in the summer, when days are longer, daylight hours are actually longer than in the winter!
Game notes: For sanity's sake, an hour is an hour, regardless of the time of year. The "longer hours in summer" is just flavour.
Long distances are also be measured in rides. When a merchant says "the city of Velprintalar is four and a half rides away", what they mean is that it takes around 25 days of travel to get there. Typically a merchant caravan will travel for 9 days straight and rest the 10th day. They make around 12 miles a day, assuming average terrain, so a ride is around 108 miles.
A coster is a group of independent merchants and carriers who have formed a loose alliance to promote and protect their business interests.