Index: usr.bin/top/machine.c
===================================================================
--- usr.bin/top/machine.c
+++ usr.bin/top/machine.c
@@ -196,7 +196,7 @@
 static const char *ordernames[] = {
 	"cpu", "size", "res", "time", "pri", "threads",
 	"total", "read", "write", "fault", "vcsw", "ivcsw",
-	"jid", "swap", "pid", NULL
+	"jid", "swap", "pid", "pbt", NULL
 };
 
 /* Per-cpu time states */
@@ -1276,6 +1276,21 @@
 		return (diff > 0 ? 1 : -1); \
 } while (0)
 
+#define ORDERKEY_PID(a, b) do { \
+	int diff = (int)b->ki_pid - (int)a->ki_pid; \
+	if (diff != 0) \
+		return (diff > 0 ? 1 : -1); \
+} while (0)
+
+#define ORDERKEY_PBT(a, b) do { \
+	time_t tdiff = b->ki_start.tv_sec - a->ki_start.tv_sec; \
+	suseconds_t udiff = b->ki_start.tv_usec - a->ki_start.tv_usec; \
+	if (tdiff != 0) \
+		return (tdiff > 0 ? 1 : -1); \
+	if (udiff != 0) \
+		return (udiff > 0 ? 1 : -1); \
+} while (0)
+
 /* compare_cpu - the comparison function for sorting by cpu percentage */
 
 static int
@@ -1420,6 +1435,42 @@
 	return (0);
 }
 
+/* compare_processid - the comparison function for sorting by pid */
+static int
+compare_processid(const void *arg1, const void *arg2)
+{
+	const struct kinfo_proc *p1 = *(const struct kinfo_proc * const *)arg1;
+	const struct kinfo_proc *p2 = *(const struct kinfo_proc * const *)arg2;
+
+	ORDERKEY_PID(p1, p2);
+	ORDERKEY_PCTCPU(p1, p2);
+	ORDERKEY_CPTICKS(p1, p2);
+	ORDERKEY_STATE(p1, p2);
+	ORDERKEY_PRIO(p1, p2);
+	ORDERKEY_RSSIZE(p1, p2);
+	ORDERKEY_MEM(p1, p2);
+
+	return (0);
+}
+
+/* compare_processbt - the comparison function for sorting by birthtime */
+static int
+compare_processbt(const void *arg1, const void *arg2)
+{
+	const struct kinfo_proc *p1 = *(const struct kinfo_proc * const *)arg1;
+	const struct kinfo_proc *p2 = *(const struct kinfo_proc * const *)arg2;
+
+	ORDERKEY_PBT(p1, p2);
+	ORDERKEY_PCTCPU(p1, p2);
+	ORDERKEY_CPTICKS(p1, p2);
+	ORDERKEY_STATE(p1, p2);
+	ORDERKEY_PRIO(p1, p2);
+	ORDERKEY_RSSIZE(p1, p2);
+	ORDERKEY_MEM(p1, p2);
+
+	return (0);
+}
+
 /* assorted comparison functions for sorting by i/o */
 
 static int
@@ -1511,7 +1562,8 @@
 	compare_ivcsw,
 	compare_jid,
 	compare_swap,
-	NULL
+	compare_processid,
+	compare_processbt
 };
 
 
Index: usr.bin/top/top.1
===================================================================
--- usr.bin/top/top.1
+++ usr.bin/top/top.1
@@ -1,5 +1,5 @@
 .\" $FreeBSD$
-.Dd October 2, 2018
+.Dd December 25, 2018
 .Dt TOP 1
 .Os
 .Sh NAME
@@ -148,12 +148,16 @@
 The default delay between updates is 1 second.
 .It Fl o Ar field
 Sort the process display area on the specified field.
-The field name
-is the name of the column as seen in the output, but in lower case:
-\*(lqcpu\*(lq, \*(rqsize\*(lq, \*(rqres\*(lq, \*(rqtime\*(lq,
-\*(rqpri\*(lq, \*(rqthreads\*(lq, \*(lqtotal\*(lq, \*(rqread\*(lq,
-\*(rqwrite\*(lq, \*(rqfault\*(lq, \*(rqvcsw\*(lq, \*(rqivcsw\*(lq,
-\*(lqjid\*(lq, \*(rqswap\*(lq or \*(rqpid\*(lq.
+The field name is the name of the column as seen in the output
+.Po except for
+.Cm pbt
+which defines the process birth time
+.Pc ,
+but in lower case:
+.Cm cpu , size , res , time , pri , threads , total , read , write , fault ,
+.Cm vcsw , ivcsw , jid , swap, pid ,
+or
+.Cm pbt .
 .It Fl p Ar pid
 Show only the process
 .Ar pid .