You don’t need to change a file.
This sets time to 1:
db getmail setprop username@externaldomain.com@imap.externaldomain.com Time 1
Then expand the template with
expand-template /etc/cron.d/getmail
Now you should see the right value in /etc/cron.d/getmail
EDIT:
If it worked, db show getmail should show a Time of 1.
Just ping me if it still doesn’t work…