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SECTION 4. HEADERS

expression:

Nbits  = |BITPIX| × GCOUNT ×

(PCOUNT + NAXIS1 × NAXIS2 × · · · × NAXISm),

(4.2)

where Nbits must be non-negative and is the number of bits excluding fill, m is the value

of NAXIS, and BITPIX, GCOUNT, PCOUNT, and the NAXISn represent the values associated

with those keywords. If Nbits > 0, then the data array shall be contained in an integral

number of 2880-byte FITS data blocks. The header of the next FITS extension in the

file, if any, shall start with the first FITS block following the data block that contains

the last bit of the current extension data array.

4.4.2

Other Reserved Keywords

The reserved keywords described below are optional, but if present in the header they

must be used only as defined in this standard. They apply to any FITS structure with

the meanings and restrictions defined below. Any FITS structure may further restrict

the use of these keywords.

4.4.2.1

General Descriptive Keywords

DATE Keyword  The value field shall contain a character string giving the date on

which the HDU was created, in the form YYYY-MM-DD, or the date and time when the

HDU was created, in the form YYYY-MM-DDThh:mm:ss[.sss. . . ], where YYYY shall be the

four-digit calendar year number, MM the two-digit month number with January given by

01 and December by 12, and DD the two-digit day of the month. When both date and

time are given, the literal T shall separate the date and time, hh shall be the two-digit

hour in the day, mm the two-digit number of minutes after the hour, and ss[.sss. . . ]

the number of seconds (two digits followed by an optional fraction) after the minute.

Default values must not be given to any portion of the date/time string, and leading

zeros must not be omitted. The decimal part of the seconds field is optional and may

be arbitrarily long, so long as it is consistent with the rules for value formats of §4.2.

The value of the DATE keyword shall always be expressed in UTC when in this

format, for all data sets created on Earth.

The following format may appear on files written before January 1, 2000. The value

field contains a character string giving the date on which the HDU was created, in the

form DD/MM/YY, where DD is the day of the month, MM the month number with January

given by 01 and December by 12, and YY the last two digits of the year, the first two

digits being understood to be 19. Specification of the date using Universal Time is

recommended but not assumed.

When a newly created HDU is substantially a verbatim copy of another HDU, the

value of the DATE keyword in the original HDU may be retained in the new HDU instead

of updating the value to the current date and time.

FITS Standard