Yes, the error message doesn’t give enough details. There are validity checks on the digital signature and one of the criteria is that it should be within the validity period of the signature and if the current date (according to the BIOS) is before the start date, it complains.
It doesn’t mean every problem relates to date. Sometimes it’s just corrupted CD/DVD. You change the date either from another system like Windows (which doesn’t support UTC so when you boot back to Linux from Windows you got bad time unless You use NTP) or in the BIOS. It’s the menu you got before you are at the boot prompt so you press Delete, F2 or F10 or even other keys to enter that and then you look for date settings (they are mostly the first option to change).