Retrieve data modified since a specific date

I am making an API call to retrieve tariff data that is entered on the platform, in my call I would like to offer users the possibility of entering a specific date from which to retrieve the data. What I manage to do for now is allow them to choose the data that has been modified and therefore entered on the platform for a certain number of days, but what I want to do is allow them to enter a specific date instead of the number of days, could you help me on this point? thanks in advance.

here is my code :

 function subDaysFromDate(date,d){
    // d = number of day ro substract and date = start date
  var result = new Date(date.getTime()-d*(24*3600*1000));
  return result
}     

//Ask the question of number of days to retrieve and retrieve data (Tarif)
//Pose la question du nombre de jours à récupérer et récupère les tarifs
function GetAllTarifs()
{   
  try{
  var ui = SpreadsheetApp.getUi(); 
  var resultat = ui.prompt('Search Tarif from a date?').getResponseText();
  //Logger.log(parseInt(resultat,10))
  if (isNaN(parseInt(resultat, 10))) {
    SpreadsheetApp.getUi().alert("You have to insert the number of days"); 
 }
 else
 {
      var nbdays=parseInt(resultat, 10)
   SpreadsheetApp.getActiveSpreadsheet().toast("Search Tarif modified since " + nbdays + " days")
   GetAllTarifsFrom(subDaysFromDate(new Date(),nbdays))
  }
 }
catch(e)
 {
    SpreadsheetApp.getUi().alert("Execution Error."+e);
}
}