From the MVA web site:
http://mva.state.md.us/VehicleServ/REG/NewtoMd.htm
Maryland Excise Titling Tax
If the vehicle has been recently purchased, the 5% excise tax is based on the greater of the total purchase price verified by a notarized MVA Bill of Sale (form VR-181), signed by both the buyer(s) and the seller(s) in which the actual price paid for the vehicle is stated or $640. In other cases, the total purchase price or the valuation shown in a national publication of used car values adopted for use by the Administration will be used to establish the tax. There is an addition or subtraction for high or low mileage. On vehicles seven years old and older, the tax is based on the greater of the purchase price or the minimum book value of $640.
Note: As a new resident, to comply with Maryland law and take advantage of a lower excise tax,
you must title and register your vehicle within 60 days of moving to Maryland.
For vehicles less than 7 years old:
Vehicle(s) titled in a state with a tax rate equal to or higher than Maryland's 5% tax rate will cost $100.
If the vehicle is registered in a state that imposes no tax, the tax will be assessed at 5% of the value of the vehicle.
Vehicles from a state with a lower tax rate than Maryland's 5% will be taxed at the rate of the difference between the two states. Example: MD 5%, VA 3%, the difference is a 2% tax rate. Minimum tax - $100. When the value of the vehicle is less than $2,000, the 5% rate will apply.
For a vehicle over 7 years old:
Tax is based on the greater of the purchase price or the minimum of $640. The MVA may require you to submit additional documentation to substantiate the purchase price.
For a leased vehicle:
Lease agreement and proof of monthly lease payments (leased vehicles only) – If you have paid or are paying excise tax to another state through your lease payments, the documents may be needed to determine the amount of excise tax credit you may receive.
On a closing note, just because you move to the state doesn't necessarily mean you have a vehicle; I went and picked up one of our vehicles 6 months after we moved here and used the date of when I brought the vehicle into the state (for the 60 day timer), not the day I moved here. With that said, just put a date down 60 days or less when registering, I had zero problems... 