WIP: added text allowing "Z" for timezone offset.

ch04.adoc

@@ -236,25 +236,24 @@ UDUNITS permits a number of alternatives to the word **`since`** in the units of
 All the alternatives have exactly the same meaning in UDUNITS.
 For compatibility with other software, CF strongly recommends that `since` should be used.
 
-The reference datetime string (appearing after the identifier **`since`**) is required.
-It may include date alone, or date and time, or date, time and time zone offset.
-Its format is __y__-__m__-__d__ [__H__:__M__:__S__ [__Z__]], where [...] indicates an optional element,
+The reference datetime string (appearing after the identifier **`since`**) is required. It must include the date, which may optionally be followed by time or time zone offset or both. Its format is __y__-__m__-__d__ [__H__:__M__:__S__ [__Z__]], where [...] indicates an optional element:
 
 * _y_ is year, _m_ month, _d_ day, _H_ hour and _M_ minute, which are all integers of one or more digits, and _y_ may be prefixed with a sign (but note that some CF calendars do not permit negative years; see <<calendar>>),
 
 * _S_ is second, which may be integer or floating point (see <<leap-seconds>> regarding __S__>59),
 
-* _Z_ is the time zone offset with respect to UTC.
-This is an interval of time, specified in one of the formats described below.
-Only numbers (digits, `+`, `-` and `:`) are allowed in _Z_, not time zone names or acronyms.
+* _Z_ is the time zone offset. This is an interval of time, specified in one of the formats described below.
+Only numbers (digits, `+`, `-` and `:`) or the letter "Z" are allowed in _Z_, not time zone names or acronyms.
 
 The default time zone offset is zero.
 In a time zone with zero offset, time (approximately) equals mean solar time for 0 `degrees_east` of longitude.
 (Although this may be exact in a model, in reality the time with zero time zone offset differs by some seconds from mean solar time; see the discussion of UTC and leap seconds in <<calendar>>.)
 If both time and time zone offset are omitted the time is 00:00:00 (the beginning of the day i.e. midnight at 0 `degrees_east`).
-Thus, **`units = "days since 1990-1-1"`** means the same as **`units = "days since 1990-1-1 0:0:0"`**.
+Thus, **`units = "days since 1990-1-1"`** means the same as **`units = "days since 1990-1-1 00:00:00"`**.
 
-The time zone offset _Z_ must be in one of the following four formats, any of which may be prefixed with a sign:
+The time zone offset _Z_ must be in one of the following five formats, any of which may be prefixed with a sign:
+
+** The letter `Z` indicating zero offset, sometimes referred to as "Zulu Time".
 
 ** _H_, the hour alone, of one or two digits e.g. **`-6`**, **`2`**, **`+11`**, which is sufficient for many time zones.
 
@@ -264,6 +263,10 @@ The time zone offset _Z_ must be in one of the following four formats, any of wh
 
 ** three digits, of which the first is the hour (0--9) e.g. **`530`**.
 
+If the time zone offset is the letter `Z` or begins with a sign, the space before it may be omitted.
+
+While the default ((of omitting the _Z_ component)) is an offset of zero, we suggest that a zero offset be specified to avoid any confusion where omitting it might be misunderstood as indicating local time.
+
 For example, **`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 a time zone where the datetime is six hours behind the default.
 Subtracting the time zone offset from a given datetime converts it to the equivalent datetime with zero time zone offset e.g. **`1989-12-31 18:00:00 -6`** identifies the same instant as **`1990-1-1 0:0:0`**.
 
@@ -400,7 +403,7 @@ When these calendars are being be used for timelines _with_ leap seconds (i.e. c
 * A datetime in the excluded range must not be used as a reference datetime e.g. **`seconds since 2016-12-31 23:59:60`** is not a permitted value for **`units`**.
 
 * The coordinate value does not count any leap seconds which occurred between the reference datetime and the datetime represented by the coordinate.
-  For instance, 60 **`seconds after 23:59:00`** always means 00:00:00 on the next day, even if there is a leap second at 23:59:60, which makes the actual interval 61 seconds between 23:59:00 and 00:00:00 on the next day.
+  For instance, 60 **`seconds after 23:59:00`** always means 0:0:0 on the next day, even if there is a leap second at 23:59:60, which makes the actual interval 61 seconds between 23:59:00 and 0:0:0 on the next day.
 
 Because of the last point, the difference between two coordinate values with the same **`units`** string does not exactly equal the length of the interval between instants they represent if there were any leap seconds between them.
 This discrepancy can happen in cases 2, 3 and 4 of the **`standard`**, **`proleptic_gregorian`**, and **`julian`** calendars.

ch07.adoc

@@ -70,7 +70,7 @@ dimensions:
 variables:
   float time(time);
     time:standard_name = "time";
-    time:units = "days since 2024-11-8 09:00:00";
+    time:units = "days since 2024-11-8 09:00:00 Z";
     time:bounds = "time_bnds";
   float time_bnds(time,nv);
 ----
@@ -330,7 +330,7 @@ variables:
     ppn:cell_methods = "time: sum";
   double time(time);
     time:long_name = "time";
-    time:units = "h since 1998-4-19 6:0:0";
+    time:units = "h since 1998-4-19 6:0:0 Z";
     time:bounds = "time_bnds";
   double time_bnds(time,nv);
 data:
@@ -400,7 +400,7 @@ variables:
     TS_var:units="K2";
     TS_var:cell_methods="time: variance (interval: 1 hr comment: sampled instantaneously)";
   float time(time);
-    time:units="days since 1990-01-01 00:00:00";
+    time:units="days since 1990-01-01 00:00:00 Z";
     time:bounds="time_bnds";
   float time_bnds(time,nv);
 data: