Cinnamaldehyde -> trans-3-phenylprop-2-enal
The rules for systematic nomenclature in organic chemistry have changed several times. They were first issued in Geneva in 1892, followed by the Liége Rules in 1930. The IUPAC rules were issued in 1957 (Sections A and B), 1969 (Sections A, B, and C), and 1979 (Sections A, B, C, D, E, F, and H). The rules were revised in the 1993 Guide to IUPAC Nomenclature of Organic Compounds and amended by official publications from 1993 through 2002. The current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) was published in December 2013.
According to the current IUPAC recommendations, the preferred IUPAC name (PIN) of cinnamaldehyde is (2E)-3-phenylprop-2-enal.

However, many compounds can have two or more systematic names in accordance with several methods recommended by IUPAC. Other names (that are not preferred IUPAC names but that are generated in accordance with IUPAC recommendations) may be used in general nomenclature. In addition, various traditional names are retained for use in IUPAC nomenclature, though the number of retained names has been reduced with each succeeding edition of the IUPAC recommendations.
According to the following subsections of the current IUPAC recommendations, the traditional name cinnamaldehyde may still be used in general nomenclature.

P-65.1.1.2 Retained names only for general nomenclature
(…)
P-65.1.1.2.2 The following names are retained for general nomenclature with
functionalization but no substitution is allowed. (…)
(…)
cinnamic acid
(…)

 

P-66.6.1.2 Names of aldehydes derived from retained names of carboxylic acids
Names of aldehydes derived from retained names are formed by changing the ‘ic acid’ or ‘oic acid’ ending of the retained names of carboxylic acids to ‘aldehyde’. (…)
(…)
P-66.6.1.2.3 Aldehydes derived from retained acid names given in P-65.1.1.2 are only used in general nomenclature; no substitution is allowed. Preferred IUPAC names are systematic names (…).