[CMake] How specify revision for ctest_update(...) do check out?

[Convey, Christian J CIV NUWC NWPT]

I'm writing a (new style) ctest script.  When I run it, I want it to check out the project's source code, as it was at the very beginning of the day (00:00).  What's the right way to do this?  (BTW, I'm using Subversion.)

I would have assumed it's done by setting CTEST_NIGHTLY_START_TIME, but I'm not 100% what that variable is for.
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