We do that as well for convenience. A rounded number gives people more margin to work when the amount of events is either too spread or too closed.
Look at ancient history books. They rarely go for exact numbers, using those same intervals between notorious events that must be studied with close attention due to the time passed.
@Exceptionally Average Individual And Lurker
Your system of measures may be easier to break down, but it is based on factors that are no less arbitrary than the imperial system.