Zapatec Calendar Week Start - Thursday

Question

One of our websites uses an old DHTML widget called Zapatec. It has a fairly straightforward setup and works as expected on my local environment and for other users. The week starts on a Monday as it has been setup.

For one of our users though, their widget always starts on a Thursday. I've been racking my brain, wondering what it could be. Could it be their browser overriding the settings?

   Zapatec.Calendar.setup({
    firstDay          : 1,
    weekNumbers       : true,
    showOthers        : true,
    showsTime         : false,
    step              : 2,
    range             : [anoinicio, 2999.12],
    electric          : false,
    singleClick       : true,
    inputField        : inputC,
    button            : botonC,
    //ifFormat          : "%d/%m/%Y",
    ifFormat          : formato_fecha,
    daFormat          : "%Y/%m/%d",
    align             : "Br",
    displayArea       : false,
    dateText          : false,
      cache :true
  });

This is my code for the calendar setup. First day is set to 1 or Monday. It is in .cfm or Coldfusion file.

How it looks for me:

How it looks for our user:

Please ignore the week numbers in the first picture, I just added it.

Any assistance on this would be appreciated. Cheers.