Green’s Dictionary of Slang

the supreme example of, the greatest version; note superlative version in cit. 1963.

1963		F. Kohner Affairs of Gidget 57: You’ve got it made in the shade. You’re making the scene with an all-time timer.
1974		P. Gent North Dallas Forty 147: [of a hangover] ‘An all-timer,’ Maxwell groaned.