It's been a long time since I took Shakespeare, so I had to look it up again to be sure. The warning from the soothsayer that Caesar's life was in danger actually covered a
30 day period ending on March 15th (so that would also include March 4th).
In the movie "Ides of March", the phase is used as a metaphor for political intrigue/drama.
title - Why it is called "The Ides of March"? - Movies & TV Stack Exchange
"According to the Roman historian Suetonius (690–130 CE), the haruspex (seeress) Spurinna warned Caesar in mid-February 44, telling him that the
next 30 days were to be fraught with peril, but the danger would end on the Ides of March. When they met on the Ides of March Caesar said "you are aware, surely, that the Ides of March have passed" and Spurinna responded, "surely you realize that they have not yet passed?""
The Ides of March (thoughtco.com)
""Exactly a month earlier Caesar had visited a soothsayer named Spurinna. who had predicted that his life would be in danger for the
next 30 days. These events were recorded by the Roman poet Publius Terentius Afer, who is now usually called Terence." "
The phrase 'Beware the Ides of March' - meaning and origin. (phrases.org.uk)