An era of intelligent machines where vs an era of intelligent machines when [duplicate]

Solution 1:

I'm not sure of exactly what rules, but based on what "feels right" to me, I use:

• era when when I'm talking about an indeterminate time that is defined by what comes after the "when", and
• era where when I'm talking about a definite time that contains whatever features come after the "where".

For example:

My parents lived in an era when newspapers reigned supreme.

My parents lived during some time period defined by when newspapers reigned supreme.

My parents lived in an era where newspapers reigned supreme.

The era my parents lived in happened to have newspapers reigning supreme.

When using the present tense ("We live in an era..."), I always use "where" because I'm talking about a definite time (now).