Both encoding/json and encoding/xml have an "omitempty" struct tag option that
omits the corresponding field if its value is "empty". That's well-defined for
builtin types (e.g. int is 0, string is ""). It doesn't work well for named
types, whether they be time.Time or something a programmer has defined in their own
program.

I propose these two packages use
  type isZeroer interface {
    IsZero() bool
  }
and use that where possible. time.Time already supports this.

I am prepared to do the work to implement this if there's consensus we should do it.