diff --git a/README.md b/README.md
index 61bb30b..c8afaf6 100644
--- a/README.md
+++ b/README.md
@@ -35,7 +35,7 @@ Please see [mal test cases](https://github.com/rhysd/Crisp/tree/master/spec/cris
 ## Development Environment
 
 - OS X
-- Crystal 0.7.1 ~ 0.7.3
+- Crystal 0.7.1 ~ 0.7.3 or 0.7.7
 
 ## License
 
diff --git a/src/crisp/core.cr b/src/crisp/core.cr
index dc050c6..988aaf0 100644
--- a/src/crisp/core.cr
+++ b/src/crisp/core.cr
@@ -339,7 +339,7 @@ module Crisp
   end
 
   def time_ms(args)
-    (Time.now.to_i.to_i32) * 1000
+    Time.now.epoch_ms.to_i32
   end
 
   # Note: