changed checksum vs changed timestamp on file creation

[Michi Mutsuzaki]

Hi,

I'm using monit (version 5.3.2 on Ubuntu 12.04) to monitor a file. The
file might not exist when monit starts, and I want to execute a
command when the file gets created. My conf file looks something like
this:

check file my_file with path /my/file
  if changed timestamp
  # if changed checksum
    then exec "/my/command"

With "changed timestamp", it works as I expect; the command gets
executed after the file is created. When I use "changed checksum"
instead, however, the command doesn't get executed, and the log says
that checksum has not changed. Is this expected? If so, is there a way
to monitor for file creation without using timestamp? I'm trying to
avoid using timestamp because this file gets touched a lot, and the
command shouldn't get executed if the file content hasn't changed.

Thanks!
--Michi