Date2DN:

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   Compute the day number from the date. Uses the format from clock. If no
   inputs are given it will compute the day number for the instant
   of the function call.

   Since version 1.
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   Form:
   dn = Date2DN( datetime )
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   ------
   Inputs
   ------
   datetime     (1,6) [year month day hour minute seconds]

   -------
   Outputs
   -------
   dn           (1,1) Day number

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   References: Montenbruck, O., T.Pfleger, Astronomy on the Personal
               Computer, Springer-Verlag, Berlin, 1991, p. 12.
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Children:

Common: Time/DTSToDTA
Common: Time/Date2JD