diff --git a/send-daily-mail.sh b/send-daily-mail.sh
index 1a76672..010d3ec 100755
--- a/send-daily-mail.sh
+++ b/send-daily-mail.sh
@@ -1,6 +1,6 @@
 #!/bin/sh
 
-yesterday=`date -d 'yesterday' --iso-8601`
+yesterday=`date -d 'yesterday' -I`
 
 echo "/tmp/webmention-storage/$yesterday"