doc en: add note about internal information of date value

doc/source/tutorial/data.txt

@@ -84,6 +84,11 @@ Date and time type
 
 The date and time type of groonga is Time. Actually, a Time column stores a date and time as the number of microseconds since the Epoch, 1970-01-01 00:00:00. A Time value can represent a date and time before the Epoch because the actual data type is a signed integer. Note that :doc:`/reference/commands/load` and :doc:`/reference/commands/select` commands use a decimal number to represent a data and time in seconds. The default value is 0.0, which means the Epoch.
 
+.. note::
+
+   Groonga internally holds the value of Epoch as pair of integer. The first integer represents the value of seconds, on the other hand, the second integer represents the value of micro seconds.
+   So, groonga shows the value of Epoch as floating point. Integral part means the value of seconds, fraction part means the value of micro seconds.
+
 The following example creates a Time column and updates existing records. The first record has the default value because not updated.
 
 .. groonga-command