Google Sheets – How to Search in Hidden Rows

To hide rows, Google Apps Script only include the methods that the OP already mentioned. The one that is able to hide several rows at a time, hideRows(rowIndex,rows) works for consecutive rows. If the rows are not consecutive, one alternative is to sort the data in advance in order to make the rows to hide to be consecutive.

This requires a script with a function onOpen(). Since you are asking about dates and not times, I'm going to break my own advice "do not use .getValue on date/time cells".

Here is a script that does what you described: grabs the dates in the first row, compares them to the present time (ignoring the potential timezone issues: UTC in Google Script vs local time in Sheets), and picks the closest. Then hides the columns to the left of the closest.

function onOpen(e) {
  var sheet = e.source.getSheetByName('Sheet1'); // or .getSheets()[0]; to apply to the first sheet.
  var width = sheet.getDataRange().getWidth();
  var dates = sheet.getRange(1, 1, 1, width).getValues().valueOf();  
  var today = Date.now();
  var minDiff = 1e9;
  var imin = 0;
  for (var i = 0; i < dates[0].length; i++) {
    if (Math.abs(dates[0][i]-today) < minDiff) {
      imin = i;
      minDiff = Math.abs(dates[0][i]-today);
    }
  }
  sheet.hideColumns(1, imin);
}

Whether some of the columns were previously hidden doesn't matter.

You may want to play it safer by hiding one column less, with sheet.hideColumns(1, imin-1); to provide a cushion against timezone discrepancy. Also, nearest date could be before or after today's; this is also worth considering.