We derive revenue from a client based on the solutions purchased by the client, the number of client employees as well as the amount, type and timing of services provided with respect to those client employees. As such, the number of client employees on our system is not a good indicator of our financial results in any period. Recurring and other revenue accounted for 97% and 99% of our total revenues during the three months ended December 31, 2019 and 2020 respectively, and 96% and 99% of our total revenues during the six months ended December 31, 2019 and 2020, respectively.
While the majority of our agreements with clients are generally cancellable by the client on 60 days’ notice or less, we have term agreements which are generally two years in length. Our agreements do not include general rights of return and do not provide clients with the right to take possession of the software supporting the services being provided. We recognize recurring fees in the period in which services are provided and the related performance obligations have been satisfied. We defer implementation fees related to our proprietary products over a period generally up to 24 months.
Interest Income on Funds Held for Clients
We earn interest income on funds held for clients. We collect funds for employee payroll payments and related taxes in advance of remittance to employees and taxing authorities. Prior to remittance to employees and taxing authorities, we earn interest on these funds through demand deposit accounts with financial institutions with which we have automated clearing house, or ACH, arrangements. We also earn interest by investing a portion of funds held for clients in highly liquid, investment-grade marketable securities.
Cost of Revenues
Cost of revenues includes costs to provide our payroll and other HCM solutions which primarily consists of employee-related expenses, including wages, stock-based compensation, bonuses and benefits, relating to the provision of ongoing client support and implementation activities, payroll tax filing, distribution of printed checks and other materials as well as delivery costs, computing costs, and bank fees associated with client fund transfers. Employee costs for recurring support are generally expensed as incurred whereas such costs for implementation of our proprietary products are capitalized and amortized over a period of 7 years. Our cost of revenues is expected to increase in absolute dollars for the foreseeable future as we increase our client base. However, we expect to realize cost efficiencies over the long term as our business scales, resulting in improved operating leverage and increased margins.
We also capitalize a portion of our internal-use software costs, which are then all amortized as Cost of revenues. We amortized $4.7 million and $5.9 million of capitalized internal-use software costs during the three months ended December 31, 2019 and 2020, respectively and $9.1 million and $11.3 million of capitalized internal-use software costs during the six months ended December 31, 2019 and 2020, respectively.
Operating Expenses
Sales and Marketing
Sales and marketing expenses consist primarily of employee-related expenses for our direct sales and marketing staff, including wages, commissions, stock-based compensation, bonuses and benefits, marketing expenses and other related costs. We capitalize certain selling and commission costs related to new contracts or purchases of additional services by our existing clients and amortize them over a period of 7 years. We pay commissions the following month after the start of service or contract signing, and bonuses for attainment of certain annual performance criteria are subsequently paid annually in the first fiscal quarter of the following year.
We will seek to grow our number of clients for the foreseeable future, and therefore our sales and marketing expense is expected to continue to increase in absolute dollars as we grow our sales organization and expand our marketing activities.
Research and Development
Research and development expenses consist primarily of employee-related expenses for our research and development and product management staff, including wages, stock-based compensation, bonuses and benefits.