dovecot - Aug 2015 - Structure of dovecot.index.pvt.log

Hi,

we are migrating from Cyrus to Dovecot and I would like to migrate seen 
flags for shared folder too.

We have Dovecot 2.2.13 prepared as:
location = 
maildir:/vmail/user/%%n/Maildir:INDEXPVT=/vmail/user/%n/Maildir/Shared/%%n

Now I'm looking for 'dovecot.index.pvt.log' syntax to be able
migrate
Seen flags. All I know is, that index contains messages UID and Seen flag.

May I ask you for a link to doc (if it exists) or into a source code? 
Should I care about 'dovecot.index.pvt.log' timestamps?

Thank you, Milo