Here's a fewconundrums that i'll offer.

The aliens are travelling towards Earth and are due to arrive at an unknown time (OP said, they are coming but it might be a month, a year Etc).

First question, to be aware that the aliens are on there way, you would have to know where they are now. To know that you must be able to track them. If you are tracking them, you know how fast they are travelling. If you know how fast they are travelling, you therefore have a good idea of their ETA.

If you say "maybe the aliens haven't left their home town yet"

Hmmm, so we would still have to know where they are at this moment. How are we (or OP) determining that these aliens are going to set out to Earth at all? Where are they receiving their intel?

Assuming that E= MC2 for them as us, and that C cannot be exceeded by them, their signals would travel at C and they could only travel at <C (99.9999999%). If we know where the signal originates, we know how long their transit time would be.

The signals being transmitted to Earth? Why are these not being received by Ham stations? Any reports of anomolous signals any place?

I could go on but you get the picture?