Is a 2018 1099 miscellaneous income, 2019 income, since we did not get the income till 2019 since it was mailed on 12/31/2018?

 (We operate on a cash basis, we received a 1099 for income that was mailed from the customer on 12/31/2018 but we did not received till Mid January 2019)

 If it is considered 2019 will not our taxes appear as we are under reporting our income?