and lower manufacturing utilization due to new products in development and the Honeywell integration. Customer service gross margin percentage declined for the three month period ended March 31, 2024 compared to the year ago quarter due to increased material costs for the repair of product lines acquired from Honeywell.
Research and development. R&D expenses were $1,031,119 an increase of $164,921, or 19.0%, in the three months ended March 31, 2024 from $866,198 in the three months ended March 31, 2023. This increase in R&D expense was the result of higher salaries and benefits due to higher headcount. As a percentage of net sales, R&D expenses decreased to 9.6% of net sales for the three months ended March 31, 2024 from 11.8% of net sales for the three months ended March 31, 2023.
Selling, general and administrative. Selling, general and administrative expenses were $2,908,193, an increase of $461,558, or 18.9%, in the three months ended March 31, 2024 from $2,446,635 in the three months ended March 31, 2023. The overall increase in selling, general and administrative expense in the quarter ended March 31, 2024 was primarily the result of increased sales and marketing costs which included the amortization expense of the customer relationships intangible asset resulting from the Honeywell Transaction of $268,500 and professional and consulting fees. As a percentage of net sales, selling, general and administrative expenses were 27.1% in the three months ended March 31, 2024 compared to 33.3% for the prior year period.
Interest expense. Interest expense was $171,470 for the three months ended March 31, 2024 resulting from borrowings under the Company’s debt facility with PNC. There was no interest expense for the three months ended March 31, 2023 as the Company had no debt during the period.
Interest income. Interest income decreased by $94,751 to $36,200 in the three months ended March 31, 2024 from $130,951 in the three months ended March 31, 2023, mainly as a result of decreased cash balances during the current year period compared to the same period in the prior year.
Other income. Other income is mainly composed of royalties earned and remained relatively unchanged in the three months ended March 31, 2024 compared to the same period in the prior year.
Income tax expense. The income tax expense for the three months ended March 31, 2024 was $325,936 as compared to an income tax expense of $310,424 for the three months ended March 31, 2023.
The effective tax rate for the three-month period ended March 31, 2024 was 21.2% and differs from the statutory tax rate primarily due to higher state taxes related to a taxable gain from the sale of the Company’s King Air aircraft.
The effective tax rate for the three-month period ended March 31, 2023 was 19.6% and differs from the statutory tax rate primarily due to an increased R&D credit, as well as permanent items and state taxes.
Net income. The Company reported net income for the three months ended March 31, 2024 of $1,208,316 as compared to net income of $1,271,103 for the three months ended March 31, 2023. On a diluted basis, the net income per share was $0.07 for the three months ended March 31, 2024 compared to net income per share of $0.07 for the three months ended March 31, 2023.
Six Months Ended March 31, 2024 Compared to the Six Months Ended March 31, 2023
Net sales. Net sales were $20,047,579 for the six months ended March 31, 2024 compared to $13,856,709 for the six months ended March 31, 2023, an increase of 44.7%. Product sales decreased $1,713,661 or 15.5% and customer service sales increased $6,869,017 or 279.6% as compared to the year ago period. The decrease in product sales for the six months ended March 31, 2024 was primarily the result of reduced shipments of displays for retrofit programs to commercial air transport customers partially offset by an increase of shipments of displays to general aviation and military customers. The increase in customer service primarily reflects customer service sales of the product lines acquired from Honeywell. EDC sales increased $1,035,514, or 282.2%, compared to the year-ago period reflecting increased EDC business.
Cost of sales. Cost of sales increased by $3,549,300, or 65.8%, to $8,942,055, or 44.6% of net sales, in the six months ended March 31, 2024, compared to $5,392,755 or 38.9% of net sales, in the six months ended March 31, 2023. The increase in cost of sales was primarily the result of an increase in customer service sales volume for the six months ended March 31, 2024 compared to the six months ended March 31, 2023. The Company’s overall gross margin was 55.4% and 61.1% for the six months ended March 31, 2024 and 2023, respectively. This decrease in overall gross margin percentage for the six months ended March 31, 2024 is primarily the