update

ch04.adoc

@@ -429,17 +429,19 @@ variables:
 data: // time coordinate variable and the date/time it represents
   time_tai = 2; // 2017-1-1 0:0:0 because no leap seconds in the timeline
   time_stdnone = 2; // 2017-1-1 0:0:0 because no leap seconds in the timeline
-  time_stdutc = 2; // 2017-1-1 0:0:1 because the leap second in the timeline is not counted
+  time_stdutc = 2; // 2017-1-1 0:0:0 because the leap second in the timeline is not counted
   time_utc = 2; // leap second 2016-12-31 23:59:60
   time_unknown = 2; // unknown whether 2016-12-31 23:59:60 or 2017-1-1 0:0:0
 ----
 
-This example shows five time coordinate variables with the same **`units`** attribute.
+This example shows five scalar time coordinate variables.
+Although they all the value 2 and the same **`units`** attribute, they do not all refer to the same date/time, as shown in the comments on their data values, for two reasons.
+First, they treat the leap second differently that was added to the UTC calendar at the end of 2016.
+Second, in **`utc`** and **`tai`** the reference date/time 2016-12-31 23:59:58 does not refer to the same instant of the real-world timeline.
 The first four of them correspond to the instants marked 2 **`seconds since 2016-12-31 23:59:58`** in <<leap-second-timelines>>.
-Although all five have the same value and reference date/time, they do not all refer to the same instant, as shown in the comments on their data values, because of their different treatments of the leap second added to the UTC calendar at the end of 2016.
 
-The value of **`2`** seconds for **`time_stdnone`**, **`time_utc`** and **`time_tai`** can be correctly interpreted as the length of the interval from the reference date/time 2016-12-31 23:59:58 to 2017-1-1 0:0:0; note, however, that in **`utc`** and **`tai`** this date/time does not refer to the same instant of the real-world timeline.
-In the case of **`time_stdutc`**, the time coordinate is again **`2`** but represents 2017-1-1 0:0:1, although the interval along the timeline between the reference date/time 2016-12-31 23:59:58 and 2017-1-1 0:0:1 is actually 3 seconds.
+The value of **`2`** seconds for **`time_stdnone`**, **`time_utc`** and **`time_tai`** can be correctly interpreted as the length of the interval from the reference date/time 2016-12-31 23:59:58 to the date/time identified.
+In the case of **`time_stdutc`**, the time coordinate again represents 2017-1-1 0:0:0, but the interval along the timeline between the reference date/time 2016-12-31 23:59:58 and 2017-1-1 0:0:0 is actually 3 seconds.
 In the case of **`time_unknown`**, we cannot convert the time coordinate to a date/time with certainty, and we do not know the exact length of the interval.
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