What does ’25 or 6 to 4′ by Chicago refer to?

American jazz rockers Chicago were building a name for themselves as one of the most exciting young bands of the late 1960s and early 1970s. With a ripping lead guitar player in Terry Kath and three distinctive vocalists backed up an emphatic horn section, Chicago was as unique as any band at the time was. But they were missing something essential: a signature song.

[@yohannbiimu]

Hmmm...I've racked my brain at times about what the heck this title meant. My mind can rest now.

[@stevea2909]

Now I know!