Why does some of the fill-in information in our pdf fields get changed during the email process?

In our event booking form, two of the fields are for 'Start Time' and 'End Time'. We have drop down boxes where my staff select the correct time for each field. When completed, we send out the pdf confirmation. I don't know if it happens with every form we send out, but we've been doing it this way for years and customers started contacting us a couple weeks ago saying the start and end times are not what they had booked. They seem to just be random times. When we look at our information, and the form that was sent to them, everything is correct.