If it's 20:51 (8:51 PM) PST and someone else has 11:51 (11:51 AM) the next day, that's a 15-hour time difference ahead of PST.
They are most likely in **Japan** (Japan Standard Time, JST), which is UTC+9 and exactly 16 hours ahead of PST during standard time, or 15 hours ahead during Pacific Daylight Time.
Other possibilities in that same time zone include:
- South Korea
- Parts of eastern Russia (like Yakutsk)
- Parts of eastern Indonesia
But Japan is the most common and likely location for that time difference.