It was a forgotten ltime->neg=0 (neg was the only forgotten variable).
I scanned field.cc for other places where we would forget to set neg, found none.
A test for the bug.


mysql-test/r/date_formats.result:
  result update
mysql-test/t/date_formats.test:
  a test for BUG#1960 "date_format() returns spurious '-' for valid dates"
sql/field.cc:
  When preparing ltime from the 3-byte date, don't forget to set ltime->neg to 0
  (otherwise it remains unitialized).
  Dates are not allowed to be negative (only times can be, when they mean a time interval),
  so it's ok to always set neg to 0.