diff --git a/man7/system_data_types.7 b/man7/system_data_types.7
index 28d276c69..49ca2d8e1 100644
--- a/man7/system_data_types.7
+++ b/man7/system_data_types.7
@@ -110,6 +110,29 @@ See also:
 .BR times (2),
 .BR clock (3)
 .RE
+.\"------------------------------------- clockid_t --------------------/
+.TP
+.I clockid_t
+.RS
+Include:
+.IR <sys/types.h> .
+Alternatively,
+.IR <time.h> .
+.PP
+Used for clock ID type in the clock and timer functions.
+According to POSIX,
+it shall be defined as an arithmetic type.
+.PP
+Conforming to:
+POSIX.1-2001 and later.
+.PP
+See also:
+.BR clock_adjtime (2),
+.BR clock_getres (2),
+.BR clock_nanosleep (2),
+.BR timer_create (2),
+.BR clock_getcpuclockid (3)
+.RE
 .\"------------------------------------- div_t ------------------------/
 .TP
 .I div_t