E0018: a name the standard reserves for a type, function or block

Data type names (DT, TOD, INT), standard function names (LT, ADD, INT_TO_REAL) and standard function block names (TON, CTU) are keywords: they “shall not be used for any other purpose, for example, variable names” (Ed.2 2.1.3 and Table C.2; Ed.3 6.1.3).

VAR dt : TIME; END_VAR       (* E0018: DT is a data type *)
VAR cycle_dt : TIME; END_VAR (* fixed *)