Squashed commit of the following:

commit 1fbebc3b3803b274e80a9adad9ba9a4015898472
Author: arunsrinivasan <aragorn168b@gmail.com>
Date:   Wed Apr 20 22:12:56 2016 +0200

    PR #1663, fix for #1658 enhance doc on wday in ?IDateTime

NEWS.md

@@ -243,6 +243,8 @@
 
  27. The default number of over-allocated spare column pointer slots has been increased from 64 to 1024. The wasted memory overhead (if never used) is insignificant (0.008 MB). The advantage is that adding a large number of columns by reference using := or set() inside a loop will not now saturate as quickly and need reallocating. An alleviation to issue [#1633](https://github.com/Rdatatable/data.table/issues/1633). See `?alloc.col` for how to change this default yourself.
 
+ 28. `?IDateTime` now makes clear that `wday`, `yday` and `month` are all 1- (not 0- as in `POSIXlt`) based, [#1658](https://github.com/Rdatatable/data.table/issues/1658); thanks @MichaelChirico.
+
 ### Changes in v1.9.6 (on CRAN 19 Sep 2015)
 
 #### NEW FEATURES

man/IDateTime.Rd

@@ -153,6 +153,8 @@ round to weeks, months, quarters, and years.
  for hour, day of year, day of week, day of month, week, month,
  quarter, and year.
  
+ These values are all taken directly from the \code{POSIXlt} representation of \code{x}, with the notable difference that while \code{yday}, \code{wday}, and \code{mon} are all 0-based, here they are 1-based.
+ 
 }
 \references{