"a week ago last thu" wrong

[gregrwm]

$ cal
      May 2015
Su Mo Tu We Th Fr Sa
                1 2
 3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
$ date +"%F %a" -d"today"
2015-05-30 Sat
$ date +"%F %a" -d"yesterday"
2015-05-29 Fri
$ date +"%F %a" -d"last thu"
2015-05-28 Thu
$ date +"%F %a" -d"7 days ago"
2015-05-23 Sat
$ date +"%F %a" -d"a week ago"
2015-05-23 Sat
$ date +"%F %a" -d"7 days ago yesterday"
2015-05-22 Fri
$ date +"%F %a" -d"a week ago yesterday"
2015-05-22 Fri
$ date +"%F %a" -d"7 days ago last thu"
2015-05-21 Thu
$ date +"%F %a" -d"a week ago last thu"
2015-05-20 Wed

shouldn't "7 days ago last thu" and "a week ago last thu" be the same?

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: coreutils 8.23-3ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-10.10-generic 3.19.2
Uname: Linux 3.19.0-10-generic i686
ApportVersion: 2.16.2-0ubuntu4
Architecture: i386
Date: Sat May 30 21:50:10 2015
InstallationDate: Installed on 2015-01-14 (136 days ago)
InstallationMedia: Lubuntu 14.10 "Utopic Unicorn" - Release i386 (20141022.1)
SourcePackage: coreutils
UpgradeStatus: No upgrade log present (probably fresh install)