included suggestions from Issue #584

ch04.adoc

@@ -236,9 +236,7 @@ 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>>),
 
@@ -265,7 +263,9 @@ The time zone offset _Z_ must be in one of the following five formats, any of wh
 
 ** three digits, of which the first is the hour (0--9) e.g. **`530`**.
 
-While the default (unspecified) 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.
+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`**.