Please do not include pixels that are intentionally different in each rendering of the frame in sessions uploaded to this renderfarm - in this case the frame date stamps.
The returned pictures are verified for similarity and frames that are different by definition will be hard to verify.

i can insert a "bar" on low part of frame doing a frame counter ?

The answer is "maybe". For full-frame rendering, if the bar is always the same for any one frame regardless of how many times that one frame is rendered and on what machine, then yes. For everything else, no.

yes, the bar is always the same on same frame

i.e :

extended : all frist frames are marked like "frame 1 of 240" and all second frames will be marked : "frame 2 of 240"

it´s ok ?

Probably maybe =)
The thing is, we cannot guarantee that all fonts are available on all machines etc. - there are a lot of issues with text in general and stamping in particular.
If you really want frame stamps then usually the best thing is to stamp it in postpro after rendering on the farm but you can give it a shot and see how it turns out.