I just answered a similar question in Timestamping responses - #3 by LN -- it's funny how people around the world seem to have the same ideas at the same time!
To summarize, you can use the audit log by adding a row of type and name audit in your form:
| survey | ||
|---|---|---|
| type | name | label |
| audit | audit |
A first pass at deeper documentation is at https://github.com/opendatakit/docs/pull/649. Note that you will need to use Aggregate 1.5+ to receive the audit logs.
If you only want to time one or two questions, you can use something like if(${age}='', '', once(format-date-time(now(), "%b %e, %Y at %H:%M:%S"))) to time when a field named age is first edited. See a full sample form here.