diff --git a/NAMESPACE b/NAMESPACE
index 2c9e072..37316b1 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -30,6 +30,8 @@ export(fixed_to_CE)
 export(fixed_to_b2k)
 export(fixed_to_julian)
 export(year_axis)
+exportClasses(GregorianCalendar)
+exportClasses(JulianCalendar)
 exportClasses(RataDie)
 exportClasses(TimeIntervals)
 exportClasses(TimeScale)
diff --git a/R/AllClasses.R b/R/AllClasses.R
index 4d3d6cb..c339f47 100644
--- a/R/AllClasses.R
+++ b/R/AllClasses.R
@@ -43,10 +43,9 @@ NULL
 #' @author N. Frerebeau
 #' @family classes
 #' @family calendar classes
-#' @family gregorian era classes
 #' @docType class
 #' @aliases GregorianCalendar-class
-#' @keywords internal
+#' @exportClass GregorianCalendar
 .GregorianCalendar <- setClass(
   Class = "GregorianCalendar",
   prototype = list(
@@ -67,7 +66,7 @@ NULL
 #' @family calendar classes
 #' @docType class
 #' @aliases JulianCalendar-class
-#' @keywords internal
+#' @exportClass JulianCalendar
 .JulianCalendar <- setClass(
   Class = "JulianCalendar",
   prototype = list(
diff --git a/R/calendar.R b/R/calendar.R
index 733b803..a17abe5 100644
--- a/R/calendar.R
+++ b/R/calendar.R
@@ -24,7 +24,7 @@ setMethod(
 )
 
 #' @export
-#' @describeIn gregorian Gregorian BP era.
+#' @rdname calendar
 BP <- function(...) {
   .GregorianCalendar(
     label = tr_("BP"),
@@ -35,7 +35,7 @@ BP <- function(...) {
 }
 
 #' @export
-#' @describeIn gregorian Gregorian b2k era.
+#' @rdname calendar
 b2k <- function(...) {
   .GregorianCalendar(
     label = tr_("b2k"),
@@ -46,7 +46,7 @@ b2k <- function(...) {
 }
 
 #' @export
-#' @describeIn gregorian Gregorian BC era.
+#' @rdname calendar
 BC <- function(...) {
   .GregorianCalendar(
     label = tr_("BC"),
@@ -56,7 +56,7 @@ BC <- function(...) {
 }
 
 #' @export
-#' @describeIn gregorian Gregorian BCE era.
+#' @rdname calendar
 BCE <- function(...) {
   .GregorianCalendar(
     label = tr_("BCE"),
@@ -66,7 +66,7 @@ BCE <- function(...) {
 }
 
 #' @export
-#' @describeIn gregorian Gregorian AD era.
+#' @rdname calendar
 AD <- function(...) {
   .GregorianCalendar(
     label = tr_("AD"),
@@ -75,7 +75,7 @@ AD <- function(...) {
 }
 
 #' @export
-#' @describeIn gregorian Gregorian CE era.
+#' @rdname calendar
 CE <- function(...) {
   .GregorianCalendar(
     label = tr_("CE"),
@@ -84,7 +84,7 @@ CE <- function(...) {
 }
 
 # @export
-# @describeIn gregorian Gregorian AUC era.
+# @rdname calendar
 # AUC <- function(...) {
 #   .GregorianCalendar(
 #     label = tr_("AUC"),
@@ -104,7 +104,10 @@ J <- function(...) {
 }
 
 # Mutators =====================================================================
-calendar_year <- function(object) object@year
+calendar_year <- function(object) {
+  if (is.null(object)) return(NULL)
+  object@year
+}
 
 ## Getters ---------------------------------------------------------------------
 #' @export
diff --git a/man/GregorianCalendar-class.Rd b/man/GregorianCalendar-class.Rd
index 3b548a2..7c6c6e3 100644
--- a/man/GregorianCalendar-class.Rd
+++ b/man/GregorianCalendar-class.Rd
@@ -25,5 +25,3 @@ N. Frerebeau
 }
 \concept{calendar classes}
 \concept{classes}
-\concept{gregorian era classes}
-\keyword{internal}
diff --git a/man/JulianCalendar-class.Rd b/man/JulianCalendar-class.Rd
index e08ac59..e3ebf72 100644
--- a/man/JulianCalendar-class.Rd
+++ b/man/JulianCalendar-class.Rd
@@ -25,4 +25,3 @@ N. Frerebeau
 }
 \concept{calendar classes}
 \concept{classes}
-\keyword{internal}
diff --git a/man/calendar.Rd b/man/calendar.Rd
index c8e360e..5c733d4 100644
--- a/man/calendar.Rd
+++ b/man/calendar.Rd
@@ -5,11 +5,29 @@
 \alias{calendar}
 \alias{calendar-method}
 \alias{calendar,character-method}
+\alias{BP}
+\alias{b2k}
+\alias{BC}
+\alias{BCE}
+\alias{AD}
+\alias{CE}
 \title{Calendar}
 \usage{
 calendar(object)
 
 \S4method{calendar}{character}(object)
+
+BP(...)
+
+b2k(...)
+
+BC(...)
+
+BCE(...)
+
+AD(...)
+
+CE(...)
 }
 \arguments{
 \item{object}{A \code{\link{character}} string specifying the abbreviated label of
diff --git a/man/gregorian.Rd b/man/gregorian.Rd
index b7eab81..12f8801 100644
--- a/man/gregorian.Rd
+++ b/man/gregorian.Rd
@@ -1,28 +1,9 @@
 % Generated by roxygen2: do not edit by hand
-% Please edit documentation in R/AllGenerics.R, R/calendar.R
+% Please edit documentation in R/AllGenerics.R
 \docType{methods}
 \name{gregorian}
 \alias{gregorian}
-\alias{BP}
-\alias{b2k}
-\alias{BC}
-\alias{BCE}
-\alias{AD}
-\alias{CE}
 \title{Gregorian Calendar}
-\usage{
-BP(...)
-
-b2k(...)
-
-BC(...)
-
-BCE(...)
-
-AD(...)
-
-CE(...)
-}
 \arguments{
 \item{...}{Currently not used.}
 }
@@ -32,21 +13,6 @@ A \code{\linkS4class{GregorianCalendar}} object.
 \description{
 Gregorian Calendar
 }
-\section{Functions}{
-\itemize{
-\item \code{BP()}: Gregorian BP era.
-
-\item \code{b2k()}: Gregorian b2k era.
-
-\item \code{BC()}: Gregorian BC era.
-
-\item \code{BCE()}: Gregorian BCE era.
-
-\item \code{AD()}: Gregorian AD era.
-
-\item \code{CE()}: Gregorian CE era.
-
-}}
 \examples{
 ## Define time scales
 calendar("BP")