I investigated further. Within the JSON returned from the World Time API, is a field:
"raw_offset":-28800
Which is the number of seconds away from UTC your current time is. The value above is for your location. You are 8 hours behind UTC. 8 hours is 28,800 seconds.
You are using the Unix Time extracted from the JSON, you would need to add 28,800 to the Unix Time to get UTC, then check if you are close to the ISS time overhead returned from the other API call.
Something like this in your getCurrentTime()
function:
currentTime = doc["unixtime"];
rawOffset = doc["raw_offset"];
// Add -ve offsets, subtract +ve offsets
// to get UTC for ISS.
currentTime += (-rawOffset);
Cheers,
Norm.