diff --git a/R/zzz.R b/R/zzz.R
index 883e725..3ef30b8 100644
--- a/R/zzz.R
+++ b/R/zzz.R
@@ -17,3 +17,4 @@
 ## }
  
 variogram <- function(object, ...) UseMethod("variogram")
+units.default <- function(x, ...) base::units.difftime(x)