we define timestamp as this:
{ "timestamp", "", "Time stamp", GDA_VALUE_TYPE_TIMESTAMP },

unfortunatly, AFAIR gda value timestamps are pretty much expected to be iso
formated, or similar, a mysql timestamp uses the format yyyymmddhhiiss
hence using to_string on it produces strange results.
a few options exist to work around this:
a) use GDA_VALUE_TYPE_STRING
b) modify fill_gda_value to convert timestamps (that are not null) to iso
format.. (by adding a few - / :'s)