If you got fired for no reason after the 120 days and you filed an action in court the burden would be on you to prove your assertion, which I would assume to be work days since that would be a longer period.
120 calendar days would average to 4 months.
Assuming a 5 day work week, 120 days would average 6 months, not counting holidays. Longer if you include holidays.
However, considering the disparity in the calculations from date to date for employees hired on different dates in the year, my 2 cents worth guesses that a court would call it calendar days so that the same amount of days would apply to each employee regardless of the date of hire.
Make sense?
That other union contracts specify calendar days would also give credence to calendar days as conventional use.

