Best answer by Mathis
This issue is simply related to your time zone.
According to our Help Center article: http://help.soundcloud.com/customer/en/portal/articles/2167322-track-or-playlist-release-date-keeps-changing-?b_id=9644
"If the release date that you have put keeps changing, this is because you are in a time zone outside of GMT. As the site is based in GMT, any information you put in will be reflected in this time zone. "
You can use this Time Zone calculator to calculate your release date to a GMT value: https://www.timeanddate.com/worldclock/converter.html
Hope you understand, apologies for any inconvenience.