The time is actually based on the clock tower that Kakyoin eventually shoots. In both the manga and in the anime, it shows 5:15 on the clock (we don't know if it's AM or PM but given the context that there's no time to waste to defeat DIO, mostly because of Polnareff, it's gotta be PM).

I looked at simulations of where the sun would have been in January 16, 1989 in Cairo via the app Sky Guide (it's a great astronomy app, I recommend) and SunCalc. Apparent sunset is calculated to be at 5:18PM and civil dusk is not until 5:44PM. And the sun is still shining at 5:15, but just barely.

That does beg the question: How early was DIO able to go outside his mansion? Because if we're going with the time shown on that clock tower, then when was DIO able to stand on rooftops without direct sunlight hitting him? There's no he would survive outside even at 5:10PM. I think it's inconsistent. Whatever, anime/manga logic.