A short comment in the BBC Crowd Science audio program Does Time really Exist?'s discussion of the slow divergence between UTC and TAI (IAT) (coordinated time and international atomic time) says that NASA and ESA for example avoid launches around leap seconds.
Is this true? Is it done out of an abundance of caution, or even an overabundance, or are there mixed clocks, some running UTC and some running IAT and offsets hard-coded in for each launch? Currently the difference is 37 seconds and there have been 27 leap seconds since 1970.
In the past it would not have been much of a burden to avoid them if a launch is considered a few minute or few hour event, but the duration of a mission is often years or decades, so an entire mission can almost never completely avoid a leap second.
So how do launches avoid leap seconds? And why?