lib/command_duration: avoid relying on a specific locale

lib/command_duration.bash

@@ -5,8 +5,12 @@
 # Get shell duration in decimal format regardless of runtime locale.
 # Notice: This function runs as a sub-shell - notice '(' vs '{'.
 function _shell_duration_en() (
-	# DFARREL You would think LC_NUMERIC would do it, but not working in my local
-	LC_ALL='en_US.UTF-8'
+	# DFARREL You would think LC_NUMERIC would do it, but not working in my local.
+	# Note: LC_ALL='en_US.UTF-8' has been used to enforce the decimal point to be
+	# a period, but the specific locale 'en_US.UTF-8' is not ensured to exist in
+	# the system.  One should instead use the locale 'C', which is ensured by the
+	# C and POSIX standards.
+	local LC_ALL=C
 	printf "%s" "${EPOCHREALTIME:-$SECONDS}"
 )