remove redundant text about the standard calendar, clarify the deprec…

…ation of year and month units

ch04.adoc

@@ -210,11 +210,9 @@ and minutes)."
 
 The acceptable units for time are listed in the UDUNITS database. The most commonly used of these strings (and their abbreviations) includes **`day`** (**`d`**), **`hour`** (**`hr`**, **`h`**), **`minute`** (**`min`**) and **`second`** (**`sec`**, **`s`**). Plural forms are also acceptable.
 
-The reference date/time string (appearing after the identifier **`since`**) may include date alone, or date and time, or date, time and time zone. The reference date/time string is required. If the time zone is omitted the default is UTC, and if both time and time zone are omitted the default is 00:00:00 UTC.
+The reference date/time string (appearing after the identifier **`since`**) is required. It may include date alone, or date and time, or date, time and time zone. If the time zone is omitted the default is UTC, and if both time and time zone are omitted the default is 00:00:00 UTC.
 
-We recommend that the unit **`year`** be used with caution. The Udunits package defines a **`year`** to be exactly 365.242198781 days (the interval between 2 successive passages of the sun through vernal equinox). __It is not a calendar year.__ Udunits includes the following definitions for years: a **`common_year`** is 365 days, a **`leap_year`** is 366 days, a **`Julian_year`** is 365.25 days, and a **`Gregorian_year`** is 365.2425 days.
-
-For similar reasons the unit **`month`**, which is defined in link:$$http://www.unidata.ucar.edu/software/udunits/$$[`udunits.dat`] to be exactly **`year/12`**, should also be used with caution.
+UDUNITS defines a **`year`** to be exactly 365.242198781 days (the interval between 2 successive passages of the sun through vernal equinox). __It is not a calendar year.__ UDUNITS defines a  **`month`** to be exactly **`year/12`**, which is __not a calendar month__. The CF standard follows UDUNITS in the definition of units, but we recommend that **`year`** and **`month`** should not be used, because of the potential for mistakes and confusion.
 
 [[time-axis-ex]]
 [caption="Example 4.4. "]
@@ -330,30 +328,6 @@ double time(time) ;
 
 ====
 
-The mixed Gregorian/Julian calendar used by Udunits is explained in the following excerpt from the udunits(3) man page: 
-----
-
-The udunits(3) package uses a mixed Gregorian/Julian  calen-
-dar  system.   Dates  prior to 1582-10-15 are assumed to use
-the Julian calendar, which was introduced by  Julius  Caesar
-in 46 BCE and is based on a year that is exactly 365.25 days
-long.  Dates on and after 1582-10-15 are assumed to use  the
-Gregorian calendar, which was introduced on that date and is
-based on a year that is exactly 365.2425 days long.  (A year
-is  actually  approximately 365.242198781 days long.)  Seem-
-ingly strange behavior of the udunits(3) package can  result
-if  a user-given time interval includes the changeover date.
-For example, utCalendar() and utInvCalendar() can be used to
-show that 1582-10-15 *preceded* 1582-10-14 by 9 days.
-	
-----
-
-
-
-Due to problems caused by the discontinuity in the default mixed Gregorian/Julian calendar, we strongly recommend that this calendar should only be used when the time coordinate does not cross the discontinuity. For time coordinates that do cross the discontinuity the **`proleptic_gregorian`** calendar should be used instead.
-
-
-
 
 [[discrete-axis, Section 4.5, "Discrete Axis"]]
 === Discrete Axis