The API for specifying TTL on a time-series collection looks like this:

db.createCollection('ts', {timeseries: {timeField: 't', expireAfterSeconds: 3600}}); 

However, to change the "expireAfterSeconds" value, users have to specify a completely different "clusteredIndex" option:

db.runCommand({collMod: 'ts', clusteredIndex: {expireAfterSeconds: 10}}) 

This is confusing and we should make the API consistent between operations.