4.	PROPERTY AND EQUIPMENT, NET

Property and equipment and accumulated depreciation and amortization were as follows:

	June 30,			December 31,
	2018			2017
Property and equipment
Buildings	$	101,109		$	60,441
Software and capitalized software costs		54,339			41,996
Computer equipment		34,176			27,928
Rental clocks		14,532			13,131
Furniture, fixtures and equipment		15,544			7,528
Leasehold improvements		1,093			767
Vehicles		50			—
		220,843			151,791
Less: accumulated depreciation and amortization		(66,200	)		(53,525	)
		154,643			98,266
Construction in progress		1,704			40,446
Land		9,023			8,993
Property and equipment, net	$	165,370		$	147,705

We capitalize computer software development costs related to software developed for internal use in accordance with ASC 350-40. For the three and six months ended June 30, 2018, we capitalized $4.6 million and $11.2 million, respectively, of computer software development costs related to software developed for internal use. For the three and six months ended June 30, 2017, we capitalized $3.5 million and $6.4 million, respectively, of computer software development costs related to software developed for internal use.

Rental clocks included in property and equipment, net represent time clocks issued to clients under month-to-month operating leases. As such, these items are transferred from inventory to property and equipment and depreciated over their estimated useful lives.

Included in the construction in progress balance at June 30, 2018 and December 31, 2017 is less than $0.1 million and $2.0 million in retainage, respectively.

We capitalize interest incurred for indebtedness related to construction of our fourth headquarters building. For the three and six months ended June 30, 2018, we incurred interest costs of $0.4 million and $0.8 million, respectively, of which we capitalized $0.3 million and $0.7 million, respectively. For the three and six months ended June 30, 2017, we incurred interest costs of $0.4 million and $0.8 million, respectively, of which we capitalized $0.1 million and $0.2 million, respectively.

Depreciation and amortization expense for property and equipment, net was $6.7 million and $12.7 million, respectively, for the three and six months ended June 30, 2018. Depreciation and amortization expense for property and equipment, net was $4.3 million and $8.2 million, respectively, for the three and six months ended June 30, 2017.