Amend with date reset causes an error

By danny0... on May 06, 2015 03:20 (imported from Google Code)

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What steps will reproduce the problem?

1. On committing, check "Amend Last Commit", "Set author date", "Reset"
2. Press "OK"

What is the expected output? What do you see instead?
"fatal: invalid date format: " is reported.

What version of TortoiseGit and msysgit are you using? On
what operating system?
TortoiseGit 1.8.14.0
Git for Windows 2.3.7
Windows 7 SP1

Please provide any additional information below.
It seems that git commit --date='' causes the error.
Maybe we should use git commit --date="$(date -R)" instead?