diff --git a/README.md b/README.md
index 5da06f7..398b1c9 100644
--- a/README.md
+++ b/README.md
@@ -195,6 +195,19 @@ An example implementation can be found in the BurnSushi/toml:
 - [Add tags](https://github.com/BurntSushi/toml/blob/master/internal/tag/add.go)
 - [Remove tags](https://github.com/BurntSushi/toml/blob/master/internal/tag/rm.go)
 
+Implementation-defined behaviour
+--------------------------------
+This only tests behaviour that's should be true for every encoder implementing
+TOML; a few things are left up to implementations, and are not tested here.
+
+- Millisecond precision (4 digits) is required for datetimes and times, and
+  further precision is implementation-specific, and any greater precision than
+  is supported must be truncated (not rounded).
+
+  This tests only millisecond precision, and not any further precision or the
+  truncation of it.
+
+
 Assumptions of Truth
 --------------------
 The following are taken as ground truths by `toml-test`:
diff --git a/tests/valid/datetime/milliseconds.json b/tests/valid/datetime/milliseconds.json
index bb896af..c49e400 100644
--- a/tests/valid/datetime/milliseconds.json
+++ b/tests/valid/datetime/milliseconds.json
@@ -1,18 +1,18 @@
 {
   "utc1": {
     "type": "datetime",
-    "value": "1987-07-05T17:45:56.123456Z"
+    "value": "1987-07-05T17:45:56.1234Z"
   },
   "utc2": {
     "type": "datetime",
-    "value": "1987-07-05T17:45:56.600000Z"
+    "value": "1987-07-05T17:45:56.6000Z"
   },
   "wita1": {
     "type": "datetime",
-    "value": "1987-07-05T17:45:56.123456+08:00"
+    "value": "1987-07-05T17:45:56.1234+08:00"
   },
   "wita2": {
     "type": "datetime",
-    "value": "1987-07-05T17:45:56.600000+08:00"
+    "value": "1987-07-05T17:45:56.6000+08:00"
   }
 }
diff --git a/tests/valid/datetime/milliseconds.toml b/tests/valid/datetime/milliseconds.toml
index a6c0590..24b81f8 100644
--- a/tests/valid/datetime/milliseconds.toml
+++ b/tests/valid/datetime/milliseconds.toml
@@ -1,4 +1,4 @@
-utc1  = 1987-07-05T17:45:56.123456Z
+utc1  = 1987-07-05T17:45:56.1234Z
 utc2  = 1987-07-05T17:45:56.6Z
-wita1 = 1987-07-05T17:45:56.123456+08:00
+wita1 = 1987-07-05T17:45:56.1234+08:00
 wita2 = 1987-07-05T17:45:56.6+08:00