diff --git a/basis/calendar/format/format-tests.factor b/basis/calendar/format/format-tests.factor
index c0768629f0..8b9885a63c 100644
--- a/basis/calendar/format/format-tests.factor
+++ b/basis/calendar/format/format-tests.factor
@@ -95,6 +95,8 @@ IN: calendar.format.tests
     }
 ] [ "2013-04-23T13:50:24" rfc3339>timestamp ] unit-test
 
+{ "2001-12-14T21:59:43.1-05:00" } [ "2001-12-14T21:59:43.1-05:00" rfc3339>timestamp timestamp>rfc3339 ] unit-test
+
 [
     T{ timestamp
         { year 2001 }
diff --git a/basis/calendar/format/format.factor b/basis/calendar/format/format.factor
index 8d8250e5e7..4b2601f4f2 100644
--- a/basis/calendar/format/format.factor
+++ b/basis/calendar/format/format.factor
@@ -133,9 +133,18 @@ M: timestamp year. ( timestamp -- )
         { +gt+ [ "+" write (write-rfc3339-gmt-offset) ] }
     } case ;
 
+! Should be enough for anyone, allows to not do a fancy
+! algorithm to detect infinite decimals (e.g 1/3)
+CONSTANT: rfc3339-precision 5
+: write-rfc3339-seconds ( timestamp -- )
+    second>> 1 mod rfc3339-precision 10^ * round >integer
+    number>string rfc3339-precision CHAR: 0 pad-head
+    [ CHAR: 0 = ] trim-tail [ "." write write ] unless-empty ;
+
 : (timestamp>rfc3339) ( timestamp -- )
     {
         YYYY "-" MM "-" DD "T" hh ":" mm ":" ss
+        write-rfc3339-seconds
         [ gmt-offset>> write-rfc3339-gmt-offset ]
     } formatted ;