There was no Hebrew word for "24 hour day" - the word "yom" (day) means the period from darkness (usually sundown) to darkness, which varies from the strict 24 hour modern day in duration (sometimes noticeably) and also in structure.
The Hebrews did not count days from midnight to midnight and different rabbinical schools had different rules for when exactly a day officially ended.
The Hebrews did not count days from midnight to midnight and different rabbinical schools had different rules for when exactly a day officially ended.*
However, I believe they used the same word, yom, when they were referring to a normal day as was used in Genesis.