I don't know what they'd do about months. I reckon the easiest thing to do them would be just not to use them, and use a week count instead. A lot of businesses on earth prefer to use week counts instead of months for planning anyway, so we already know that approach works. For medium-term planning, you can divide the sol-weeks of the Earth year into four quarters. (The Earth-year contains 50-51 sol-weeks, so that divides into quarters of 12-13 sol-weeks–some quarters would be 7 sols longer than others.)