The overall 12.8% increase in revenue outpaced the 5.5% increase in cost of revenue, and gross profit increased by $30.6 million, or 24.8%, as we continued to leverage our investment in people and processes to realize economies of scale.
Cloud recurring services gross margin was 68.3% for the six months ended June 30, 2018, compared to 61.8% for the six months ended June 30, 2017. The increase in Cloud recurring services gross margin reflects an increase in the proportion of Dayforce customers live for more than two years, which increased from 57% as of June 30, 2017 to 61% as of June 30, 2018, and was also attributable to consistent configuration that enabled us to realize economies of scale in customer support and hosting costs. Bureau recurring services gross margin was 70.5% for the six months ended June 30, 2018, compared to 72.2% for the six months ended June 30, 2017. Professional services and other gross margin was (54.3)% for the six months ended June 30, 2018, improving from (110.5)% for the six months ended June 30, 2017, reflecting an increase in profitable postgo-live professional services and productivity improvements in implementing new customers.
Selling, general, and administrative expense. Selling, general, and administrative expense increased $32.3 million for the six months ended June 30, 2018, compared to the six months ended June 30, 2017. Of the total $32.3 million increase, $23.2 million was attributable to expenses associated with our IPO and debt refinancing, including $11.3 million in sponsor management termination fees, $6.0 million inIPO-related share-based compensation expense, $3.7 million in other IPO transaction costs, and $2.2 million in restructuring consulting expenses. ExcludingIPO-related items, selling, general, and administrative expense increased $9.1 million, primarily due to sales commissions expense, employee benefit related costs, and increased costs associated with being a public company.
Other (income) expense, net. For the six months ended June 30, 2018, we generated $2.8 million of other income, net, compared to $3.1 million of other expense, net, for the six months ended June 30, 2017. The other income and expense, net, for the six months ended June 30, 2017, and 2018, respectively, were primarily related to remeasurement gains and losses on intercompany receivables or payables denominated in foreign currencies.
Operating profit.Operating profit increased $4.2 million, to $16.0 million for the six months ended June 30, 2018, from $11.8 million for the six months ended June 30, 2017. This $4.2 million increase was primarily attributable to an increase in gross profit, partially offset by $23.2 million inIPO-related selling, general, and administrative expense and $2.1 million in other increased share-based compensation expense related to the IPO.
Interest expense. Interest expense for the six months ended June 30, 2018, was $65.6 million, compared to $43.4 million for the six months ended June 30, 2017. This increase was primarily due to the refinancing of our debt, which resulted in a loss on extinguishment of debt of $25.7 million, partially offset by a decrease in interest expense of $4.5 million upon extinguishment of the Senior Notes, which represents one less month of interest on the Senior Notes as compared to the prior year. A 100 basis point increase in LIBOR rates would result in an approximately $7 million increase in our interest expense over the ensuing twelve-month period. Please refer to Note 8, “Debt,” for further discussion.
Income tax expense. For the six months ended June 30, 2018, we incurred income tax expense of $6.7 million, compared to $2.9 million for the six months ended June 30, 2017. The increase is attributable to the tax law changes that became effective in 2018 as part of the 2017 U.S. Tax Reform legislation, offset by a reduction in the amount of valuation allowance required to be recorded against our deferred tax assets.
(Loss) income from discontinued operations.As a result of the LifeWorks Disposition, the financial results of the LifeWorks business have been included within discontinued operations for all periods presented. For the six months ended June 30, 2018, loss from discontinued operations was $11.8 million, compared to the six months ended June 30, 2017, income from discontinued operations was $0.5 million. The loss from discontinued operations for the six months ended June 30, 2018, is primarily related to income tax expense incurred as a result of the LifeWorks Disposition.
Net loss attributable to Ceridian. Net loss attributable to Ceridian was $67.6 million for the six months ended June 30, 2018, compared to $34.1 million for the six months ended June 30, 2017, primarily due to the expenses incurred related to our IPO and debt refinancing transactions, partially offset by an increase in gross profit.
Adjusted EBITDA. Adjusted EBITDA increased by $23.4 million for the six months ended June 30, 2018, compared to the six months ended June 30, 2017, and Adjusted EBITDA margin increased to 21.1% in 2018 from 16.6% in 2017.
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