There is no extending a 14hr duty day, that's not in the regulations. Company errands and requests, like training, are considered part of the duty day, basically anything the company asks you to do is part of your duty day, you also have an obligation to record it as such. They must be providing 10hrs of rest. There are typically 2 different duty positions, a "defined duty day", maximum 14hrs, or "8 hours of flight in 24 with 10hr rest look-back". The second does NOT have a defined duty day, but does have a defined required rest period. There are many legal interpretation/opinions on this. Start with Berry 2009.