on the 15th "of" next month

Hello Alan, Mister Micawber, Beeesneees, Mordant, Esl_Expert and other native English speakers,

taipeitimes.com/News/taiwan/ … 2003399302
Lin said that Uni applied to withdraw the Taipei-Kaohsiung flight last month, and the company originally planned to stop operations starting on the 15th of next month, but the CAA asked the company to start canceling flights in March instead, he said.

  • on the 15th of next month
  • on the 15th next month

Are both fine?

In speech, yes; in writing, I think ‘of’ is necessary.

Thank you, MM.