When you purchase equipment for your business, you will be able to take the cost off as depreciation. The question is when does that cost come off. There are at least 5 options. Some let you have the deduction sooner. Your tax accountant may tell you what works best for your situation.
Section 179 allows you to take the full amount of the purchase off in the year you started using the equipment. So, if you purchase a truckmount now and start using it by Dec 31st, you could deduct the full depreciation from your 2017 tax return. The current maximum amount for this is $250,000 worth of equipment purchased in a year.
Other forms of depreciation spread the cost over 3, 5, or 7 years. The cost may be spread evenly over those years or there may be more depreciation up-front and less in later years. Think of how much value a new car loses the day you drive it off a lot. These methods are known as declining balance or double declining balance formula. I think there are a few others as well.
Most commonly depreciation is figured by the years you own something. Own it for just part of a year and you may still do the calculations as though you owned it all year. But if most of your purchases were made later in the year, and you don't use the section 179 method, your depreciation is calculated by months. You start on the 15th of a month (mid-month) in doing these calculations.