Is there a way to track the date the pending payment record was created once payment has been completed?
There isn't a field on contribution that stores when the Contribution was created v/s payment added to the contribution. But you can get such information from financial transactions table.
When a pending contribution is created a account receivable(IR) entry is created in financial transaction table and later a new entry is created with income account when a payment is received. But AR transactions are not displayed on UI when you click on contribution amount(which loads list of payments).
The only place you can see them is while creating accounting batches.