In the Double-Month Calendar, the year is divided into 6 sesters (alias sextals or bimesters) or double-months, each of which contains a whole number of weeks in a pattern of 9:9:8. The leap week is appended to the last sextal, that means only the third sextal has just 8 weeks then. The ISO-like date format is ±YYYY-S-W-D. The calendar is compatible with that standard as far as possible, e.g. weeks begin on Monday.