But, the night time and daytime describes a 24 hour day and that's how genesis gives the days of creation. So the meaning of the word Yom, though it can depict an era of course, is described and clarified in the verse itself with day and night.
What the modern term of Yom today, it is indeed, used as a 24 hours period.
There is clear explanation, that the word "Yom" is called to time of light, and "Layla" to time of darkness.
After that, we learn that "ערב" and "בקר" are describing a day.
So there are several meanings to the word "Yom".
Time of Day.
"Evening" to "Morning".
The word "Evening" have several meanings and same for "Morning".
So each of them should be examined. its a bit more complex, but I can elaborate more if you wish.
But it is clear that "Yom" represents more than what we call today "Yom".
In the Jewish belief, the modern day starts at the lack of light. based on the words "Evening and Morning" to emphasize that exact phrase.
(That's why the sabbath is kept from Friday).