@mattias: hi. thanks for reporting the problem. unfortunately i cannot reproduce this problem here. im also fairly sure that this script doesnt try to modify the displaying of the dates in any way.
if you are still having the same problem could you please post a list of any other scripts/extensions that you have installed? thank you.

Mattkolb has made a very nice addition to this script. it actually works very well. unfortunately due to some sort of formatting problem, it wont work if you directly copy and paste it. his addition is ideal for those who frequently use the 'Quick Add' option. the script with his addition is posted below. just copy this entire script over the original source.

      // ==UserScript==
      // @name Enhance Google Cal
      // @namespace http://exstodot.blogspot.com
      // @description Cleans up Google Calendar. To be used with Enhance Google script
      // @include http://www.google.com/calendar/*
      // @include https://www.google.com/calendar/*
      // ==/UserScript==
      
      //-------------------------------------- USER OPTIONS ------------------------------------------//
      // 1 means YES, 0 means NO
      
      const HIDE_PRINT = 1;
      const REARRANGE_NAV_BAR = 1;
      const QUICK_ADD_AT_TOP = 1;
      
      //-------------------------------------- DONT CHANGE ANYTHING BELOW THIS LINE ------------------------------------------//
      
      var divs = document.evaluate('/html/body/div', document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
      for (var i = 0; i < divs.snapshotLength; i++) 
      {
      	divs.snapshotItem(i).style.display = 'none';
      }
      
      if (REARRANGE_NAV_BAR) 
      {
      	var cal = document.getElementById('mothertable').firstChild.firstChild;
      	var nav = cal.firstChild.cloneNode(true);
      	
      	if (QUICK_ADD_AT_TOP) 
      	{
      		var quickadd = document.getElementById('open-quick-add').parentNode;
      		var t1 = document.getElementById('t1');
      		var t1tr = t1.firstChild.firstChild;
      		t1tr.appendChild(quickadd);
      	}
      	
      	cal.removeChild(cal.firstChild);
      	cal = cal.parentNode;
      	
      	var tr = document.createElement('tr');
      	
      	tr.appendChild(nav);
      	cal.appendChild(tr);
      }
      
      if (HIDE_PRINT) 
      {
      	document.getElementById('printlink').parentNode.removeChild(document.getElementById('printlink'));
      }
      
      

Updated to work with https://...