diff --git a/lib/growth/calc/age.ex b/lib/growth/calc/age.ex
index d5fe58d..a057dac 100644
--- a/lib/growth/calc/age.ex
+++ b/lib/growth/calc/age.ex
@@ -3,6 +3,8 @@ defmodule Growth.Calc.Age do
   Calculate the age in months.
   """
 
+  @type precision :: :day | :week | :month | :year
+
   # NOTE: (jpd): based on [WHO instructions][0]
   # [0]: https://cdn.who.int/media/docs/default-source/child-growth/child-growth-standards/indicators/instructions-en.pdf
   @day_in_month 30.4375
@@ -10,16 +12,22 @@ defmodule Growth.Calc.Age do
   @day_in_week 7.0
 
   @doc """
-  Calculate the age with the precision of day, or week, or month. For age in weeks or months, considers completed ones, removing decimal value.
+  Calculate the age with the precision of `:day`, or `:week`, or `:month`, considering the measurement date as today.
 
-  If only date of birth is available, assumes that measurement date is today.
+  For age in weeks or months, considers completed ones, removing decimal value.
   """
-  @spec calculate(:day | :week | :month, Date.t(), Date.t()) :: pos_integer()
+  @spec calculate(precision(), Date.t(), Date.t()) :: pos_integer()
 
   def calculate(precision, date_of_birth) do
     calculate(precision, date_of_birth, Date.utc_today())
   end
 
+  @doc """
+  Calculate the age with the precision of `:day`, or `:week`, or `:month`.
+
+  For age in weeks or months, considers completed ones, removing decimal value.
+  """
+
   def calculate(:month, date_of_birth, date_of_measurement) do
     :day
     |> calculate(date_of_birth, date_of_measurement)