Macros are defined using expressions of the form:
name = constexpr;
Where name follows the normal rules for variables in programming
languages, ie. alphabetic optionally followed by alphanumerics.
constexpr must be a constant expression, either a string
(enclosed in double quotes) or an arithmetic expression
optionally followed by a scale factor.
Macros are expanded when their name, prefixed by a dollar ($)
appears in an expression, and macros may be nested within a
constexpr string.
The following reserved macro names are understood.
minute Current minute of the hour.
hour Current hour of the day, in the range 0 to 23.
day Current day of the month, in the range 1 to 31.
month Current month of the year, in the range 0 (January) to 11
(December).
year Current year.
day_of_week
Current day of the week, in the range 0 (Sunday) to 6
(Saturday).
delta Sample interval in effect for this expression.
Dates and times are presented in the reporting time zone (see
description of -Z and -z command line options above).
