How to declare a type which is a union of string enum values?

Question

Suppose we have a string-valued enum:

export enum SomeEnum {
    Date = "date",
    DateTime = "datetime"
}

How to declare a union type based on the enum member values, i.e.

export type SomeType = ...; // "date" | "datetime"

so that we would be able to do the following:

const a: SomeType = "date";     // Ok
const b: SomeType = "datetime"; // Ok
const c: SomeType = "c";        // Compile error