Last time i checked, In world war 1 they knew on the minute that the cease fire was decleared
That's because the armistice was signed 6 hours in advance of its effect.
And even then, some didn't get the message:
"The last casualty of the war was a German, Lieutenant Thomas, who, after 11 am, was walking towards the line to inform Americans who had not yet been informed of the Armistice that they would be vacating the buildings behind them." -wiki