OP:
I heard of "a trick" you might try with that Yosemite installer that's throwing up the error message. I can't confirm that it works, or not. You'll have to try it and get back to us.
That trick is:
1. Go to the "date and time" preference pane and UNCHECK the option to set date and time automatically.
2. Now, manually "set back" your Mac's clock to, say, 2012, 2013, something like that.
3. NOW try to run the OS installer
Does this change anything?
Doing this will HURT NOTHING, and if it doesn't work, just re-enable automatic date and time (which you need to do anyway AFTER the OS install)