Calendar history
Islamic Calendar Seasonal Drift
The Islamic calendar stays lunar, so its months move through the Gregorian seasons instead of being pulled back by leap months.
A lunar year of twelve synodic months is roughly 10.88 days shorter than the tropical year, so Islamic calendar months cycle through solar seasons over time.
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The arithmetic drift
Twelve synodic lunar months add to about 354.37 days. A tropical solar year is about 365.24 days. The shortfall is roughly 10.88 days per solar year, so lunar dates appear earlier in the Gregorian seasonal year each cycle.
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Why no leap month is the point
The Islamic calendar does not solve this by adding a leap month to stay seasonal. That makes it a clear example of a lunar calendar rather than a lunisolar one. Civil and observed implementations can still vary by authority, location and sighting practice.
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CalculationTime angle
The Lunar Debt Calendar shows the drift visually, while ChronoSyncline and the Multi-Calendar Visualizer keep the displayed tabular or browser calendar label separate from official religious observance.
Related CalculationTime Tools
Open the calculators behind the story
Lunar Debt Calendar
10.88-day drift and seasonal wrap visual
ChronoSyncline Calendar
Tabular Islamic layer with visible limits
Multi-Calendar Visualizer
Browser Intl Islamic labels beside other calendars
Moon Phase Calendar
Lunar phase cycle context