The following tables provide details of selected consolidated balance sheet items:

Activity in the allowance for credit losses related to accounts receivable was as follows:


Balance at June 30, 2021	$	800
Charged to expense	103
Write-offs	(97)
Balance at December 31, 2021	$	806

Capitalized internal-use software and accumulated amortization were as follows:


	June 30, 2021		December 31, 2021
Capitalized internal-use software	$	150,922	$	171,226
Accumulated amortization	(105,904)		(118,119)
Capitalized internal-use software, net	$	45,018	$	53,107

Amortization of capitalized internal-use software costs is included in Cost of revenues and amounted to $5,882 and $6,087 for the three months ended December 31, 2020 and 2021, respectively, and $11,268 and $12,215 for the six months ended December 31, 2020 and 2021.


	June 30, 2021		December 31, 2021
Office equipment	$	5,211	$	5,709
Computer equipment	45,420		50,883
Furniture and fixtures	13,104		12,791
Software	6,641		7,494
Leasehold improvements	46,814		47,192
Time clocks rented by clients	5,399		6,039
Total	122,589		130,108
Accumulated depreciation	(62,754)		(67,683)
Property and equipment, net	$	59,835	$	62,425

Depreciation expense amounted to $4,014 and $3,974 for the three months ended December 31, 2020 and 2021, respectively, and $8,019 and $7,816 for the six months ended December 31, 2020 and 2021, respectively.

The following table summarizes changes in goodwill during the six months ended December 31, 2021:


	December 31, 2021
Balance at June 30, 2021	$	33,650
Additions attributable to acquisition	34,676
Balance at December 31, 2021	$	68,326

The Company’s amortizable intangible assets and estimated useful lives are as follows:


	June 30, 2021		December 31, 2021		Weighted average useful life (years)
Proprietary technology	$	6,129	$	27,329	6.6
Client relationships	19,200		22,300		7.8
Non-solicitation agreements	1,600		1,600		3.1
Trade names	440		1,640		5.0
Total	27,369		52,869
Accumulated amortization	(14,342)		(17,694)
Intangible assets, net	$	13,027	$	35,175

Amortization expense for acquired intangible assets was $940 and $2,000 for the three months ended December 31, 2020 and 2021, respectively, and $1,784 and $3,352 for the six months ended December 31, 2020 and 2021 respectively.

Future amortization expense for acquired intangible assets as of December 31, 2021 is as follows:


Remainder of fiscal 2022	$	3,981
Fiscal 2023	7,808
Fiscal 2024	6,803
Fiscal 2025	5,748
Fiscal 2026	4,129
Thereafter	6,706
Total	$	35,175


	June 30, 2021		December 31, 2021
Accrued payroll and personnel costs	$	73,969	$	46,764
Operating lease liabilities	7,549		6,450
Deferred revenue	9,442		11,792
Other	12,149		14,501
Total accrued expenses	$	103,109	$	79,507