I have trouble distinguishing between the sound of and <três horas>. Could these be added to the examples in pronouncing diphthongs?
It is difficult for me, too but at least here in the Lisbon area, many people pronounce 13 with an extra syllable almost like tray-uz-ee.
Olá, @david.sturge! When pronounced fast, “três horas” might sound like “tre[z]oras”, but “treze horas” would sound more like “trez[ee]horas”. This is not quite about diphthongs, but about natural pronunciation variations that happen in Portuguese when the S consonant is followed by vowels, and when the E vowel is followed by another vowel, respectively. We’ve got it covered in this Learning Note, with several examples, which might help you : Pronunciation Variations | Practice Portuguese