Del. Code Ann. tit. 30, § 1907
The tax shall be first effective with respect to income earned subsequent to December 31, 1957.
30 Del. C. 1953, § 1920; 51 Del. Laws, c. 298; 51 Del. Laws, c. 315, § 16; 68 Del. Laws, c. 187, §§ 12, 14