calc: Date Conversions
8.5.1 Date Conversions
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The ‘t D’ (‘calc-date’) [‘date’] command converts a date form into a
number, measured in days since Jan 1, 1 AD. The result will be an
integer if DATE is a pure date form, or a fraction or float if DATE is a
date/time form. Or, if its argument is a number, it converts this
number into a date form.
With a numeric prefix argument, ‘t D’ takes that many objects (up to
six) from the top of the stack and interprets them in one of the
following ways:
The ‘date(YEAR, MONTH, DAY)’ function builds a pure date form out of
the specified year, month, and day, which must all be integers. YEAR is
a year number, such as 1991 (_not_ the same as 91!). MONTH must be an
integer in the range 1 to 12; DAY must be in the range 1 to 31. If the
specified month has fewer than 31 days and DAY is too large, the
equivalent day in the following month will be used.
The ‘date(MONTH, DAY)’ function builds a pure date form using the
current year, as determined by the real-time clock.
The ‘date(YEAR, MONTH, DAY, HMS)’ function builds a date/time form
using an HMS form.
The ‘date(YEAR, MONTH, DAY, HOUR, MINUTE, SECOND)’ function builds a
date/time form. HOUR should be an integer in the range 0 to 23; MINUTE
should be an integer in the range 0 to 59; SECOND should be any real
number in the range ‘[0 .. 60)’. The last two arguments default to zero
if omitted.
The ‘t J’ (‘calc-julian’) [‘julian’] command converts a date form
into a Julian day count, which is the number of days since noon (GMT) on
Jan 1, 4713 BC. A pure date is converted to an integer Julian count
representing noon of that day. A date/time form is converted to an
exact floating-point Julian count, adjusted to interpret the date form
in the current time zone but the Julian day count in Greenwich Mean
Time. A numeric prefix argument allows you to specify the time zone;
Time Zones. Use a prefix of zero to suppress the time zone
adjustment. Note that pure date forms are never time-zone adjusted.
This command can also do the opposite conversion, from a Julian day
count (either an integer day, or a floating-point day and time in the
GMT zone), into a pure date form or a date/time form in the current or
specified time zone.
The ‘t U’ (‘calc-unix-time’) [‘unixtime’] command converts a date
form into a Unix time value, which is the number of seconds since
midnight on Jan 1, 1970, or vice-versa. The numeric result will be an
integer if the current precision is 12 or less; for higher precision,
the result may be a float with (PRECISION−12) digits after the decimal.
Just as for ‘t J’, the numeric time is interpreted in the GMT time zone
and the date form is interpreted in the current or specified zone. Some
systems use Unix-like numbering but with the local time zone; give a
prefix of zero to suppress the adjustment if so.
The ‘t C’ (‘calc-convert-time-zones’) [‘tzconv’] command converts a
date form from one time zone to another. You are prompted for each time
zone name in turn; you can answer with any suitable Calc time zone
expression (Time Zones). If you answer either prompt with a
blank line, the local time zone is used for that prompt. You can also
answer the first prompt with ‘$’ to take the two time zone names from
the stack (and the date to be converted from the third stack level).
Info Catalog
calc: Date Arithmetic
calc: Date Arithmetic
calc: Date Functions