doc/timetable: Document `normalise` fractional behaviour

diff --git a/doc/timetable.md b/doc/timetable.md
index 845c7875ddb0db006566e59d220b0daf7928f151..49fe046b5a494fa7bc04e635b1152737b47898aa 100644 (file)
--- a/doc/timetable.md
+++ b/doc/timetable.md
@@ -39,9 +39,17 @@ Returns a new independent instance of an existing timetable object.
 
 ### `:normalise ( )`
 
-Mutates the current object's time and date components so that they lie within 'normal' 
+Mutates the current object's time and date components so that are integers within 'normal'
 ranges e.g. `month` is `1`-`12`; `min` is `0`-`59`
 
+First, fractional parts are propagated down.  
+e.g. `.month=6.5` `.day=1` (which could be read as "the first day after the middle of June")
+normalises to `.month=2` `.day=16`
+
+Second, any fields outside of their normal ranges are propagated up  
+e.g. `.hour=10` `.min=100` (100 minutes past 10am)
+normalises to `.hour=11` `.min=40`
+
 
 ### `:rfc_3339 ( )` and `__tostring` metamethod