Once specified, a "calendar design" cannot be altered without becoming a new design. If a proposed design is sufficiently close to the legacy one, i.e. compatible with it, a "reform" of the local "calendar system" is possible without disruption. Examples of this include the changes to the Chinese calendar due to problems between regions in China and practical changes in religious calendars such as the Islamic calendar where the start of the month is now derived from astronomical data rather than sightings by religious leaders.