diff --git a/src/libsystemd/sd-event/sd-event.c b/src/libsystemd/sd-event/sd-event.c
index e6744989ffd..b2c2074e817 100644
--- a/src/libsystemd/sd-event/sd-event.c
+++ b/src/libsystemd/sd-event/sd-event.c
@@ -3677,8 +3677,8 @@ static int arm_watchdog(sd_event *e) {
         assert(e->watchdog_fd >= 0);
 
         t = sleep_between(e,
-                          e->watchdog_last + (e->watchdog_period / 2),
-                          e->watchdog_last + (e->watchdog_period * 3 / 4));
+                          usec_add(e->watchdog_last, (e->watchdog_period / 2)),
+                          usec_add(e->watchdog_last, (e->watchdog_period * 3 / 4)));
 
         timespec_store(&its.it_value, t);