Dictionary.net

Numerals. They are either Roman, made with letters of the Alphabet, for example, MIDCCLXXVI; or they are Arabic, as follows, 1776.

Roman figures may be used in contracts and law proceedings, and they will be held valid; but Arabic figures, probably owing to the case with which they may be counterfeited, or. altered, have been holden not to be sufficient to express the sum due on a contract; but, it seems, that if the amount payable and due on a promissory note be expressed in figures or ciphers, it will be valid. Story on Bills, §42, note; Story, Prom. Notes, §21. Indictments have been set aside because the day or year was expressed in figures. 13 Vin Ab. 210; 1 Ch. Rep. 319; S. C. 18 Eng. Com. Law Rep. 95.

Bills of exchange, promissory notes, cheeks and agreements of every description, are usually dated with Arabic figures; it is, however, better to date deeds and other formal instruments, by writing the words at length. Vide l Ch. Cr. L. 176; 1 Verm. R. 336; 5 Toull. n. 336; 4 Yeates, R. 278; 2 John. R. 233; 1 How. Mis. 256; 6 Blackf., 533.