What does '25 or 6 to 4' mean?

Well, of course it’s drug slang. I want to know what, exactly, it does mean.

What was Chicago thinking when they wrote this?

According to VH1’s “Behind the Music” it means 25 or 26 minutes to 4 o’clock. To make it easier to sing, they changed it to 25 or 6 to 4. No drug slang.

And just for kicks, SDSTAFF Songbird on 25 or 6 to 4.