Revise section 4, include new subsection on leap seconds, define utc and tai calendars and leap_seconds in units_metadata

appa.adoc

@@ -386,8 +386,8 @@ In cases where there is a strong constraint on dataset size, it is allowed to pa
 | **`units_metadata`**
 | S
 | C, D, BI
-| <<units>>
-| Specifies the interpretation (on-scale, difference or unknown) of the unit of temperature appearing in the **`units`** attribute.
+| <<units>>, and <<time-coordinate>>
+| Specifies the interpretation of a unit of measure appearing in the **`units`** attribute.
 
 | **`valid_max`**
 | N

bibliography.adoc

@@ -10,6 +10,7 @@ Sponsored by the "Cooperative Ocean/Atmosphere Research Data Service," a NOAA/un
 - [[[FGDC]]]  link:$$https://www.fgdc.gov/standards/projects/FGDC-standards-projects/metadata/base-metadata/v2_0698.pdf$$[Content Standard for Digital Geospatial Metadata].
 Federal Geographic Data Committee, FGDC-STD-001-1998.
 - [[[IEEE_754]]]  link:$$https://doi.org/10.1109/IEEESTD.2019.8766229$$[IEEE Standard for Floating-Point Arithmetic], in _IEEE Std 754-2019 (Revision of IEEE 754-2008)_, 22 July 2019.
+- [[[ISO_8601]]]  link:$$https://www.iso.org/obp/ui/#iso:std:iso:8601:-1:ed-1:v1:en$$[ISO 8601-1:2019, Date and time -- Representations for information interchange].
 - [[[Kou21]]]  link:$$https://doi.org/10.5194/gmd-14-377-2021$$[A note on precision-preserving compression of scientific data]. Kouznetsov, R.: _Geosci. Model Dev._, 14, 377-389, 2021.
 - [[[KRD21]]]  link:$$https://doi.org/10.1038/s43588-021-00156-2$$[Compressing atmospheric data into its real information content]. Klöwer, M., Razinger, M., Dominguez, J. J., Düben, P., and Palmer, T. N.: _Nat. Comput. Sci._, 1, 713-724, 2021.
 - [[[NetCDF]]]  link:$$https://doi.org/10.5065/D6H70CW6$$[NetCDF Software Package].  UNIDATA Program Center of the University Corporation for Atmospheric Research.

ch01.adoc

@@ -71,6 +71,8 @@ The netCDF utilities ncdump and ncgen use this format (see
 link:$$https://docs.unidata.ucar.edu/nug/current/_c_d_l.html$$[NUG section on CDL syntax]).
 All examples in this document use CDL syntax.
 
+calendar:: A CF calendar defines an ordered set of valid datetimes with integer seconds.
+
 cell:: A region in one or more dimensions whose boundary can be described by a set of vertices.
 The term __interval__ is sometimes used for one-dimensional cells.
 
@@ -81,6 +83,8 @@ The coordinate values must be in strict monotonic order (all values are differen
 Missing values are not allowed in coordinate variables.
 To avoid confusion with coordinate variables, CF does not permit a one-dimensional string-valued variable to have the same name as its dimension.
 
+datetime:: The set of numbers which together identify an instant of time, namely its year, month, day, hour, minute and second, where the second may have a fraction but the others are all integer.
+
 grid mapping variable:: A variable used as a container for attributes that define a specific grid mapping.
 The type of the variable is arbitrary since it contains no data.
 

ch02.adoc

@@ -176,7 +176,7 @@ If it is observational, **`source`** should characterize it (e.g., "**`surface o
 
 **`history`**:: Provides an audit trail for modifications to the original data.
 Well-behaved generic netCDF filters will automatically append their name and the parameters with which they were invoked to the global history attribute of an input netCDF file.
-We recommend that each line begin with a timestamp indicating the date and time of day that the program was executed.
+We recommend that each line begin by indicating the date and time of day that the program was executed.
 
 **`references`**:: Published or web-based references that describe the data or methods used to produce it.
 

ch03.adoc

@@ -165,6 +165,12 @@ A standard name contains no whitespace and is case sensitive.
 canonical units:: Representative units of the physical quantity.
 Unless it is dimensionless, a variable with a **`standard_name`** attribute must have units which are physically equivalent (not necessarily identical) to the canonical units, possibly modified by an operation specified by the standard name modifier (see below and <<standard-name-modifiers>>) or by the **`cell_methods`** attribute (see <<cell-methods>> and <<appendix-cell-methods>>) or both.
 
+Units of time coordinates (<<time-coordinate>>), whose **`units`** attribute includes the word **`since`**, are _not_ physically equivalent to time units that do not include **`since`** in the **`units`**.
+To mark this distinction, the canonical unit given for quantities used for time coordinates is **`s since 1958-1-1`**.
+The reference datetime in the canonical unit (the beginning of the day i.e. midnight on 1st January 1958 at 0 `degrees_east`) is not restrictive; the time coordinate variable's own **`units`** may contain any reference datetime (after **`since`**) that is valid in its calendar.
+(We use `1958-1-1` because it is the beginning of International Atomic Time, and a valid datetime in all CF calendars; see also <<leap-seconds>>.)
+In both kinds of time **`units`** attribute (with or without **`since`**), any unit for measuring time can be used i.e. any unit which is physically equivalent to the SI base unit of time, namely the second.
+
 description:: The description is meant to clarify the qualifiers of the fundamental quantities such as which surface a quantity is defined on or what the flux sign conventions are.
 We don't attempt to provide precise definitions of fundumental physical quantities (e.g., temperature) which may be found in the literature.
 The description may define rules on the variable type, attributes and coordinates which must be complied with by any variable carrying that standard name (such as in Example 3.5).