Hi Tomeu Rosselló,
just found the answer to your problem in
https://github.com/moodle/moodle/blob/master/mod/data/field/date/field.class.php
and in
https://github.com/moodlehq/moodleapp/blob/master/src/addons/mod/data/fields/date/services/handler.ts.
You can manipulate a date field with a url like that:
https://<mainurl>/webservice/rest/server.php?
wstoken=<token>&
wsfunction=mod_data_update_entry&
moodlewsrestformat=json&
entryid=<eintryid>&
data[0][fieldid]=<fieldid>&
data[0][subfield]=year&
data[0][value]=2000&
data[1][fieldid]=<fieldid>&
data[1][subfield]=month&
data[1][value]=1&
data[2][fieldid]=<fieldid>&
data[2][subfield]=day&
data[2][value]=5&
data[3][fieldid]=<fieldid>&
data[3][subfield]=z&
data[3][value]=1
Documentation is not really satisfying ... .
Cheers corvus albus