On the website alongside the match there is a sidebar stating that the “date and time to be confirmed”. Once the match is confirmed, the sidebar disappears.
On a ticket printed at home, the time of the match is shown next to the match day (Jornada) and season (Temporada) if this has been confirmed.
If it has not been confirmed, a reference date appears with a note indicating that the definitive time and date have yet to be confirmed.
It is always recommendable to check the time and date of the match at http://www.fcbarcelona.com/