There are several answers to that question based on your relationship to the contract, prime contractor, sub-contractor or owner all can have different requirements make sure you review division 1 of your contract manual. Additionally the specifics of the cause and what may be required in your specific contract can vary. If a CPM critical path method schedule was prepared as part of the contract then you would do a review of the critical path commonly known as a TIA time impact analysis reviewing the original plan of the work compared to actual work as a result of the change or delay this is required in many public works or commercial contracts. If the time extension is due to weather you would submit based on the impact it had on the work reviewing the normal weather patterns compared to actual. As an example this January, we have experienced a large amount of rain in a short time this could justify a time extension based only on the fact your crews could not work effectively, then showing what is normal in the month of jan. compared to what has actually happened. These types of claims are generally referred to as force majeure events and can sometimes limit your options with regards to extensions of time.