clarification of time coordinate requirements

ch03.adoc

@@ -165,6 +165,11 @@ 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 choice of reference time and date (midnight on 1st January 1958) is arbitrary and not restrictive; the time coordinate variable's own **`units`** may contain any reference time and date (after **`since`**) that is valid in its calendar.
+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).

ch04.adoc

@@ -185,8 +185,12 @@ The `computed_standard_name` attribute indicates that the values in variable
 [[time-coordinate, Section 4.4, "Time Coordinate"]]
 === Time Coordinate
 
-Variables representing reference time must always explicitly include the **`units`** attribute; there is no default value.
-The **`units`** attribute takes a string value that follows the formatting requirements of the <<UDUNITS>> package. These requirements can best be described by an example with explanatory comments:
+A time coordinate identifies an instant along the continuous physical dimension of time, whether in reality or a model.
+Variables containing time coordinates must always explicitly include the **`units`** attribute.
+The **`units`** attribute takes a string value that follows the formatting requirements of the <<UDUNITS>> package.
+It must comprise a unit of measure that is physically equivalent to the SI base unit of time (i.e. the second), followed by the word **`since`** and a reference date-time. 
+There is no default value for the **`units`**.
+These requirements can best be described by an example with explanatory comments:
 
 The time unit specification **`seconds since 1992-10-8 15:15:42.5 -6:00`** indicates seconds since October 8th, 1992  at  3 hours,  15 minutes  and  42.5 seconds in the afternoon in the time zone which is six hours to the west of Coordinated Universal Time (i.e.  Mountain Daylight Time).
 The time zone specification can also be written without a colon using one or two digits (indicating hours) or three or four digits (indicating hours and minutes).

history.adoc

@@ -7,6 +7,7 @@
 
 === Working version (most recent first)
 
+* {issues}166{Issue #166}: Clarify that time coordinate variables must have **`units`** containing **`since`** and a reference time; distinguish between canonical units of time with and without **`since`**.
 * {issues}530{Issue #530]: Define "the most rapidly varying dimension", and use this phrase consistently with the clarification "(the last dimension in CDL order)".
 * {issues}163[Issue #163]: Provide a convention for boundary variables for grids whose cells do not all have the same number of sides.
 * {issues}174[Issue #174]: A one-dimensional string-valued variable must not have the same name as its dimension, in order to avoid its being mistaken for a coordinate variable.