Index: share/man/man9/time.9
===================================================================
--- share/man/man9/time.9
+++ share/man/man9/time.9
@@ -31,7 +31,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 17, 2004
+.Dd May 4, 2021
 .Dt TIME 9
 .Os
 .Sh NAME
@@ -49,6 +49,15 @@
 The
 .Va boottime
 variable holds the system boot time.
+This time is set initially when the system boots.
+When the current time is set, adjusted by ntpd, or when a new time is read when
+the system resumes,
+.Va boottime
+is adjusted so that boottime + uptime reflects the new current time.
+The
+.Xr sysctl 8
+.Va kern.boottime
+returns this value.
 .Pp
 The
 .Va time_second
@@ -83,6 +92,7 @@
 The
 .Va boottime
 variable may be read and written without special precautions.
+It is adjusted when the phase of the system time changes.
 .Sh SEE ALSO
 .Xr clock_settime 2 ,
 .Xr ntp_adjtime 2 ,
Index: sys/kern/kern_tc.c
===================================================================
--- sys/kern/kern_tc.c
+++ sys/kern/kern_tc.c
@@ -103,11 +103,18 @@
 volatile time_t time_second = 1;
 volatile time_t time_uptime = 1;
 
+/*
+ * The system keeps track of it's current time by adding the system uptime to
+ * the boot time. The timehands track this boot time so that time is consistent
+ * acround changes to time via clock_settime, settimeofday, and smaller steps
+ * from ntpd. It's also reset after a suspended system resumes. Boottime vaires
+ * over the life of the system as these events unfold.
+ */
 static int sysctl_kern_boottime(SYSCTL_HANDLER_ARGS);
 SYSCTL_PROC(_kern, KERN_BOOTTIME, boottime,
     CTLTYPE_STRUCT | CTLFLAG_RD | CTLFLAG_MPSAFE, NULL, 0,
     sysctl_kern_boottime, "S,timeval",
-    "System boottime");
+    "System boottime.");
 
 SYSCTL_NODE(_kern, OID_AUTO, timecounter, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
     "");