Frequently Asked Questions
What is a Julian Day Number?
It is a continuous count of days since noon Universal Time on January 1, 4713 BC. Because it is a single running number, you can subtract any two dates without calendar arithmetic.
What is the Modified Julian Date?
The Modified Julian Date is the Julian Date minus 2,400,000.5. It is a smaller number that starts at midnight, which is convenient for modern records.
Why does the Julian Date day start at noon?
The convention dates to astronomy, where starting the count at noon kept a full night of observations within a single date. The fractional part of the Julian Date is measured from noon.
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