You can concatenate them as regular strings, e.g. in an AttributeManager:

@Value(my_date_attribute) @Value(my_time_attribute)

Thank you for answering. they are not strings. the formats are date and time and will be translated to DateTime.

Thats fine, internally, FME treats datetimes as strings.

In fact datatypes really only exist when data is read/written - oversimplification, some more advanced use cases mean you need to change/specify data types within the process. In your case, datetimes in FME can just be treated as strings