I repaired a MacBook Pro which had been powered off for an extended time and couldn't load OS X onto it. Turned out it was using hardware time to determine if a cert was valid when validating the OS X install image. (Validation is done by the downloaded installer so I infer that it was trying to make sure the download was okay and was not trying to prevent third-party OS installs). I fixed it by spawning a shell from the installer and setting the date the unix way -- but what is a mere civilian supposed to do?
The ability of modern hardware to download install an OS directly from firmware is a truly great and useful modern innovation, but it leaves a lot of unanswered questions: What if my network is bad? What if this used hardware is stolen? What if I don't have an account? What if Apple goes bankrupt?
USB/SD/DVD installers do not require network access nor any sort of account/purchase verification, and can be created (relatively) easily and officially from downloaded installers. I have bootable disk images for every OS X installer since OS X Mavericks and I highly recommend it.
I'll include a USB installer whenever I sell an old laptop. It costs me next to nothing and gives the buyer some reassurance that they're not buying a brick. And if I decide to hang on to a piece of hardware, I have some assurance that I'll be able to use it 10 years from now.
Still not known or obvious to many, but less cryptic than a terminal to most: reset the NVRAM (aka PRAM). Hold command-option-P-R keys until you hear the startup chime for a second time.
Ah, of course not, since how can the default be set to the current time?
I almost always install after booting to a full OS X system on an external hard drive (so NTP syncs the time), either via a downloaded installer or after logging in to the wifi captive portal (since Internet Recovery won't do WPA Enterprise).
On Linux and Windows, the NTP update daemon "sanity checks" the new date. It refuses to update the time if the new time is "too far" from the current time. Is the Mac any different on this front?
"Mere" is a bit much. You're not an immortal superhero.
To answer your question: a non-technologist pays someone like you to fix it. Much like the way most people bring their cars to mechanics instead of spending hundreds of hours learning to fix the vehicles personally. This type of specialization is prerequisite to the large civilizations in which we live.
In this case fixing the issue required knowledge of how certs work in order to guess that there is a date issue. That's probably pretty rare at fix-your-computer shops.
It's pure luck that I'd seen this type of issue before because I'm one of those mere civilians: my Windows machine at work loses BIOS settings on power down. I've seen that until the date is restored, DropBox and several other things don't work.
