is_date_format() tests whether an Excel number format string
would a resolve to a date in Excel. For example, the number format string
"yyyy-mm-dd" would resolve to a date, whereas the string "0.0\%" would
not.
This is useful if a cell formula contains a number formatting string (e.g.
TEXT(45678,"yyyy")), and you need to know that the constant 45678 is a date
in order to recover it at full resolution (rather than parsing the character
output "2025" as a year).
It is used internally to convert the value of a cell to the correct data
type.