diff --git a/entries/overview.md b/entries/overview.md
index 5f0d508..49591d6 100644
--- a/entries/overview.md
+++ b/entries/overview.md
@@ -59,8 +59,8 @@ versions:
 * **`0Y`** - Zero-padded year - 06, 16, 106
 * **`MM`** - Short month - 1, 2 ... 11, 12
 * **`0M`** - Zero-padded month - 01, 02 ... 11, 12
-* **`WW`** - Short week (since start of year) - 1, 2, 33, 52
-* **`0W`** - Zero-padded week - 01, 02, 33, 52
+* **`WW`** - Short week (since start of year) - 0, 1, 2, 33, 52, 53
+* **`0W`** - Zero-padded week - 00, 01, 02, 33, 52, 53
 * **`DD`** - Short day - 1, 2 ... 30, 31
 * **`0D`** - Zero-padded day - 01, 02 ... 30, 31
 
@@ -76,6 +76,25 @@ state which one.
 [gregorian]: https://en.wikipedia.org/wiki/Gregorian_calendar
 [utc]: https://en.wikipedia.org/wiki/Coordinated_Universal_Time
 
+The definition of `WW`/`0W` in the above list is intentionally left
+ambiguous.
+
+* There are at least four common week numbering schemes: The 
+  [ISO][iso-week] and the [US Broadcast][us-week] standards as well
+  as the output of `%W` and `%U` in many implementations of `strftime`
+  which produce numbers in the range `[0-52]`. 
+* It would be tedious to determine which system a given project
+  actually uses.
+* The last week of the year can span multiple years, so that a
+  version such as `2020.53` may have been published in 2021
+  if it was published on Friday the 1st of January 2021.
+  
+[iso-week]: https://en.wikipedia.org/wiki/ISO_week_date
+[us-week]: https://en.wikipedia.org/wiki/Broadcast_calendar
+
+Suffice it to say, there is perhaps a good reason that only few
+projects use week numbers in their versioning.
+
 # Case studies
 
 CalVer has quite a few users. These are projects selected for their