I want to start at 1 for the next quarter, but it is currently 13 days before the end of this quarter. Logically I could start auto-enter serial at -13 and incrememtn by 1. So the first record would be -13, then -12 etc.

Doesn't seem to like it. First record is -13, second is -14.

Is this a bug, a feature, or known behaviour?

By design.  It treats the "-" as a dash and will increment in the number that comes after.

3 minutes ago, Wim Decorte said:

 It treats the "-" as a dash

This is to enable you to use serial "numbers" such as "ABC-0001".

 

48 minutes ago, Wayne Irvine said:

it is currently 13 days before the end of this quarter. Logically I could start auto-enter serial at -13 and incrememtn by 1.

Logically, it makes sense only if you create exactly one record each day. So perhaps you could use the date as a unique identifier? Or pre-create the 13 records until the start of the quarter? In any case, these identifier should be meaningless, so starting a quarter at #14 should not matter.