How about this, assuming the dates are in column B of Table 1. The MATCH function returns 3, the location of 9/30, which is the date before the match. Add 1. Then use INDEX to get the value of cell 4.
INDEX(Table 1::B,MATCH(NOW(),Table 1::B)+1)
use this AppleScript ▼
--finds next next blank (or non-blank) cell in column
--select starting cell, and run
property searchFor : missing value
tell application "Numbers"
set t to front document's active sheet's first table whose selection range's class is range
tell t's selection range
set currentRow to its first cell's row's address
set searchCol ...
You can assign you custom format to an empty cell. If you then enter a value that contains a hyphen (-) or a slash (/), such as "1/21/2019", Numbers assumes it’s a date and assigns it your custom date and time format automatically. And keeps it.
Otherwise, if you already have a number in a cell or enter value that is a number, such as "20171214", Numbers ...