If you have not used the Fixed Asset Manager (an add-in program which is included with Premier Accountant Edition and all editions of Enterprise Solutions), it is definitely time to check it out! The Fixed Asset Manager calculates depreciation on many different basis. All versions of QuickBooks have the Fixed Asset List for tracking the information as part of the accounting process, and are then synced with the Fixed asset Manager.
I used to use the Fixed Asset List because it provided an easy way to track the serial number, when the warranty expired, and any other notes specific to that particular asset. But since the Fixed Asset Manager previously only calculated depreciation annually, I suggested to my clients that they just get the depreciation schedule for the next year from their tax preparer/tax accountant (if I was not doing their tax return for them already).
Now, the Fixed Asset Manager allows you to enter the date for the depreciation entry and the desired depreciation basis. It will then calculate the amount accordingly. You can edit the entry, if needed, before you actually post if to QuickBooks. This will permit including a depreciation entry as part of the month end entries which has been updated for asset purchases and sales.