When sourcing R code files, the timestamp is not the modification stamp of the source file.
The issue occurs in:
R version 2.14.0 Patched (2011-12-06 r57834)
R Under development (unstable) (2011-12-06 r57834)
(tested on win and linux)
The timestamp was correct in older versions (2.13).
I think the bug was introduced in the function source when the call srcfile was replaced with srcfilecopy.
Fixed in R-patched and R-devel.
(tested in R 2.14 r57860)