Hi Pierre,
what did you do to know that the second line gives this error? Executing the second line alone will not work as far as I understand HCI.
I guess that your target field is of data type <TIMESTAMP>. The second line alone is of type <INTEGER>. Assigning and <INTEGER> to a <TIMESTAMP> probably does not work and gives you the error message you mention.
However, executing the first line together with the second line should work. The first line is of type <DATE> and HCI can add/subtract integers to/from dates. The result is of type <DATE> and this can be converted in <TIMESTAMP>.
For the third line I'm clueless and unfortunately I have no HCI available at the moment to do a quick test.
Regards,
Volker