Mody, that's a good suggestion.
Square brackets [FIELD] are still used for VBScript expressions, for all data types.
Exclamation points: !FIELD! are used for Python expressions.
Note, you may want to explicitly set the parser (this defaults to VBScript, which will fail if the tool is run in 64 bit).
http://desktop.arcgis.com/en/arcmap/10.4/tools/data-management-toolbox/calculate-field.htm