Given the fact you need to reset the system to get it to boot up leads me to beleave your issue is a dead PRAM battery. Here is the IFIXIT Guide to get to it: [guide|994|iMac Intel 20" EMC 2133 and 2210 PRAM Battery Replacement]
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The static reset is what we call SMC reset on a Mac. Here's a good Apple T/N that explains it: [https://support.apple.com/en-us/HT201295|Reset the System Management Controller (SMC) on your Mac]. Here's a good T/N on the PRAM: [https://support.apple.com/en-us/HT204063|How to reset NVRAM on your Mac]
To prove it make sure your system is not connected to the internet as you don't want it to access the Apple time server. Now set the data & time correctly manually. Safe the settings then create a file to make sure the date & time stamp are correct. Now shut down the system give it some time and restart when you get back to the desktop make a new file it's date & Time will be back to the default.
Here's a good write-up: [http://osxdaily.com/2016/05/07/fix-mac-showing-wrong-time-date/|Fix a Mac Showing the Wrong Time & Date]
Given the fact you need to reset the system to get it to boot up leads me to beleave your issue is a dead PRAM battery. Here is the IFIXIT Guide to get to it: [guide|994|iMac Intel 20" EMC 2133 and 2210 PRAM Battery Replacement]
To prove it make sure your system is not connected to the internet as you don't want it to access the Apple time server. Now set the data & time correctly manually. Safe the settings then create a file to make sure the date & time stamp are correct. Now shut down the system give it some time and restart when you get back to the desktop make a new file it's date & Time will be back to the default.
Here's a good write-up: [http://osxdaily.com/2016/05/07/fix-mac-showing-wrong-time-date/|Fix a Mac Showing the Wrong Time & Date]