Help - My first project. Control Charts showing negative limits

Hello,

I’m after some advice. I’m trying to introduce control charts around my Companies core processes ( Public Sector). I’m green belt trained and currently have a opportunity to sell the Six Sigma principles, so I need to ensure my data and measurement is spot on.

I am measuring cycle time at present (Query Resolution), my sample is the correct size according the population, but is not normally distributed. The data skews to the left.

My Control Chart lower limit is negative in value, so how can the cycle time be negative hours ?

I Apologise if this is simple, but I want to get this right in order to capitalise on this window I have of management buy in.