diff --git a/artiq/coredevice/core.py b/artiq/coredevice/core.py
index 92f5f5739..096fbbd16 100644
--- a/artiq/coredevice/core.py
+++ b/artiq/coredevice/core.py
@@ -176,7 +176,7 @@ class Core:
 
         :param seconds: time (in seconds) to convert.
         """
-        return int64(seconds//self.ref_period)
+        return round64(seconds/self.ref_period)
 
     @portable
     def mu_to_seconds(self, mu: int64) -> float: