פירוש על עבודה זרה 76:14

This baraita teaches that if the cooking process was begun by the Jew, the non-Jew would be allowed to continue the cooking. The food is prohibited only if the non-Jew begins the cooking process.
Again we can see how lenient the rabbis were with regard to this prohibition. My impression is that amoraic rabbis inherited a series of prohibitions that were created during a period in which there was a real fear of assimilation. Many of these prohibitions seem likely to have been created as early as the Second Temple period, when Hellenism was at its height. By amoraic times Jewish identity had more fully coalesced and those Jews who remained within the fold were not threatened by eating food cooked by non-Jews. Thus they could be lenient with regard to these rules.

Sweet dates can be eaten raw. Therefore they are permitted even when cooked by a non-Jew. Bitter dates cannot be eaten raw and therefore they are certainly prohibited. The question was asked about dates that are moderately bitter and then improve in taste when cooked. The answer is that they are prohibited.