The following clocktable line:
#+BEGIN: clocktable :tstart "<-7d>" :tend "<now>"

#+END:

errors out on the tstart & tend times
According to all the documentation I can find, these are both valid
'special' time strings.
Attempts to evaluate the clocktable with C-c C-c give the error:

Updating dynamic block `clocktable' at line 57...
org-parse-time-string: Not a standard Org-mode time string: <-7d>

however this:
#+BEGIN: clocktable :tstart "<2013-05-08 Wed>" :tend "<2013-05-09 Thu>"
#+END:

works just fine


-- 
Subhan Michael Tindall | Software Developer
| smt@rentrakmail.com
RENTRAK | www.rentrak.com | NASDAQ: RENT