diff --git a/internal/inventory/btime.go b/internal/inventory/btime.go
new file mode 100644
index 0000000..58d22d8
--- /dev/null
+++ b/internal/inventory/btime.go
@@ -0,0 +1,19 @@
+package inventory
+
+import (
+	"strconv"
+	"strings"
+)
+
+// parseBtime reads the boot time, in Unix seconds, from /proc/stat. The
+// control plane compares it with the moment it asked for a reboot: a report
+// is only proof the host restarted if its boot time is later.
+func parseBtime(stat string) int64 {
+	for _, line := range strings.Split(stat, "\n") {
+		if rest, ok := strings.CutPrefix(line, "btime "); ok {
+			v, _ := strconv.ParseInt(strings.TrimSpace(rest), 10, 64)
+			return v
+		}
+	}
+	return 0
+}
diff --git a/internal/inventory/btime_test.go b/internal/inventory/btime_test.go
new file mode 100644
index 0000000..349db6d
--- /dev/null
+++ b/internal/inventory/btime_test.go
@@ -0,0 +1,16 @@
+package inventory
+
+import "testing"
+
+func TestParseBtime(t *testing.T) {
+	stat := "cpu  1 2 3 4\ncpu0 1 2 3 4\nintr 1\nctxt 99\nbtime 1757800000\nprocesses 5\n"
+	if got := parseBtime(stat); got != 1757800000 {
+		t.Fatalf("got %d", got)
+	}
+}
+
+func TestParseBtimeMissing(t *testing.T) {
+	if got := parseBtime("cpu 1 2 3\n"); got != 0 {
+		t.Fatalf("got %d, want 0", got)
+	}
+}
diff --git a/internal/inventory/collect_linux.go b/internal/inventory/collect_linux.go
index d2bbeb0..4150189 100644
--- a/internal/inventory/collect_linux.go
+++ b/internal/inventory/collect_linux.go
@@ -12,6 +12,7 @@ import (
 )
 
 func collect(r *pb.InventoryReport, includeStatic bool) {
+	r.BootTimeUnix = parseBtime(readProc("/proc/stat"))
 	r.CPU.UsagePct = cpuUsage()
 	r.CPU.Load1 = load1()
 	memTotal, memAvail, swapTotal, swapFree := meminfo()
diff --git a/internal/inventory/collect_windows.go b/internal/inventory/collect_windows.go
index 5dea7ac..34b93b9 100644
--- a/internal/inventory/collect_windows.go
+++ b/internal/inventory/collect_windows.go
@@ -20,6 +20,8 @@ var (
 )
 
 func collect(r *pb.InventoryReport, includeStatic bool) {
+	// DurationSinceBoot returns milliseconds since boot and does not wrap.
+	r.BootTimeUnix = time.Now().Add(-windows.DurationSinceBoot()).Unix()
 	r.CPU.UsagePct = cpuUsage()
 	// Windows has no load average. Left at zero; the UI already treats it as
 	// optional because it is omitempty on the wire.