I propose the declaration of the following format constants to the time package:

const (
    DateTime = "2006-01-02 15:04:05"
    DateOnly = "2006-01-02"
)

Analyzing all Go code on the module proxy (as of 2022-04-01), here's a table of all common time formats sorted in popularity order:

As can be seen, "2006-01-02 15:04:05" and "2006-01-02" are the 3rd and 4th most popular format constants. They are more popular than most of the other time format constants that did get a named declaration.

Given their popularity, I propose declaring:

const (
    DateTime = "2006-01-02 15:04:05"
    DateOnly = "2006-01-02"
)

Other constant name suggestions welcome.