How is the due date for landing gear replacement determined? I want to know how calculate the due date of Boeing MD-83 Landing gear replacement (it is every 10 year or 20000 FC). 

Is it calculated from the installed date or shop visit date?
To my knowledge, all lifed components are tracked from installed date. That is from personal experience, but I can certainly have a look through the regs to find an official source, if you require.