setAdvertisting serviceData data length max?

the code you have should work, but you're supplying an array with one element, so it'll only be sending one byte (0..255) worth of data.

You can do:

NRF.setAdvertising({0xFFFF : [buttonEpoch>>24,buttonEpoch>>16,buttonEpoch>>8,buttonEpoch]});

which will transmit 4 bytes that can be reconstructed into a 32 bit int. Normally you'd have ... &255 there but it's sort of implicit since only the bottom 8 bits of each element is used anyway.

If you want something that looks a bit nicer you can use DataView instead of all the bit shifting too.