That’s true, but still you need to use at least one RX call to start the counter, and this is what I meant that you cannot make TX-only app 
Also it should be noted, that timestamps returned by getHardwareTime() method available in SoapySDR API are actually cached at the moments when you receive samples. Which means that if you won’t periodically use RX calls during operation, you won’t be aware of current estimate of hardware time, and the only way to know it to some extent is to observe late packet indicators, mentioned by IgnasJ.
At least that was the case few months ago, when I was testing it, it is possible (but not likely) that things have changed since then.