diff --git a/FAQ.md b/FAQ.md
index 4e7d784..8650ead 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -175,7 +175,7 @@ Filling out the `tm_wday` member of `struct tm` with a `weekday` is also easy:
 tm.tm_wday = (wd - Sunday).count();
 ```
 
-Indeed, `tm_wday`'s specification says:  "days since Sunday — [0, 6]"
+Indeed, `tm_wday`'s specification says:  "days since Sunday — [0, 6]."  This formulation works, even if the implementation is encoding `weekday` with [1, 7], because `weekday` difference is specified to be circular (modulo 7).
 
 As a demonstration of the lowered importance of weekday encoding in calendrical algorithms, see [how to print a calendar](https://github.com/HowardHinnant/date/wiki/Examples-and-Recipes#calendar) which is configurable on the first day of the week.  For example here is how to use this code to make Thursday the first day of the week:
 ```c++