diff --git a/cmd-attach-session.c b/cmd-attach-session.c
index 356bd4aa..a2ae49cb 100644
--- a/cmd-attach-session.c
+++ b/cmd-attach-session.c
@@ -136,6 +136,7 @@ cmd_attach_session(struct cmd_q *cmdq, const char *tflag, int dflag, int rflag,
 		status_timer_start(c);
 		notify_attached_session_changed(c);
 		session_update_activity(s, NULL);
+		gettimeofday(&s->last_attached_time, NULL);
 		server_redraw_client(c);
 		s->curw->flags &= ~WINLINK_ALERTFLAGS;
 	} else {
@@ -181,6 +182,7 @@ cmd_attach_session(struct cmd_q *cmdq, const char *tflag, int dflag, int rflag,
 		status_timer_start(c);
 		notify_attached_session_changed(c);
 		session_update_activity(s, NULL);
+		gettimeofday(&s->last_attached_time, NULL);
 		server_redraw_client(c);
 		s->curw->flags &= ~WINLINK_ALERTFLAGS;
 
diff --git a/cmd-new-session.c b/cmd-new-session.c
index fa4f1553..7687398e 100644
--- a/cmd-new-session.c
+++ b/cmd-new-session.c
@@ -278,6 +278,7 @@ cmd_new_session_exec(struct cmd *self, struct cmd_q *cmdq)
 		status_timer_start(c);
 		notify_attached_session_changed(c);
 		session_update_activity(s, NULL);
+		gettimeofday(&s->last_attached_time, NULL);
 		server_redraw_client(c);
 	}
 	recalculate_sizes();
diff --git a/cmd-switch-client.c b/cmd-switch-client.c
index 8bc8a3c5..10171018 100644
--- a/cmd-switch-client.c
+++ b/cmd-switch-client.c
@@ -129,6 +129,7 @@ cmd_switch_client_exec(struct cmd *self, struct cmd_q *cmdq)
 	c->session = s;
 	status_timer_start(c);
 	session_update_activity(s, NULL);
+	gettimeofday(&s->last_attached_time, NULL);
 
 	recalculate_sizes();
 	server_check_unattached();
diff --git a/format.c b/format.c
index 20cf0adb..c401fa35 100644
--- a/format.c
+++ b/format.c
@@ -872,6 +872,13 @@ format_defaults_session(struct format_tree *ft, struct session *s)
 	format_add(ft, "session_created", "%lld", (long long) t);
 	format_add(ft, "session_created_string", "%s", format_time_string(t));
 
+	t = s->last_attached_time.tv_sec;
+	if (t != 0) { /* zero if never attached */
+		format_add(ft, "session_last_attached", "%lld", (long long) t);
+		format_add(ft, "session_last_attached_string", "%s",
+		    format_time_string(t));
+	}
+
 	t = s->activity_time.tv_sec;
 	format_add(ft, "session_activity", "%lld", (long long) t);
 	format_add(ft, "session_activity_string", "%s", format_time_string(t));
diff --git a/server-fn.c b/server-fn.c
index cddd762f..27908fde 100644
--- a/server-fn.c
+++ b/server-fn.c
@@ -422,6 +422,7 @@ server_destroy_session(struct session *s)
 			status_timer_start(c);
 			notify_attached_session_changed(c);
 			session_update_activity(s_new, NULL);
+			gettimeofday(&s_new->last_attached_time, NULL);
 			server_redraw_client(c);
 		}
 	}
diff --git a/tmux.1 b/tmux.1
index 8e07fc2a..a2f6cb96 100644
--- a/tmux.1
+++ b/tmux.1
@@ -3401,6 +3401,8 @@ The following variables are available, where appropriate:
 .It Li "session_activity_string" Ta "" Ta "String time of session last activity"
 .It Li "session_created" Ta "" Ta "Integer time session created"
 .It Li "session_created_string" Ta "" Ta "String time session created"
+.It Li "session_last_attached" Ta "" Ta "Integer time session last attached"
+.It Li "session_last_attached_string" Ta "" Ta "String time session last attached"
 .It Li "session_group" Ta "" Ta "Number of session group"
 .It Li "session_grouped" Ta "" Ta "1 if session in a group"
 .It Li "session_height" Ta "" Ta "Height of session"
diff --git a/tmux.h b/tmux.h
index 0223d857..8cd83e27 100644
--- a/tmux.h
+++ b/tmux.h
@@ -981,6 +981,7 @@ struct session {
 	int		 cwd;
 
 	struct timeval	 creation_time;
+	struct timeval	 last_attached_time;
 	struct timeval	 activity_time;
 	struct timeval	 last_activity_time;