The
lastModified
time is represented in milliseconds since the epoch. So all you need is the current time and the difference in the same format.
For 15 minutes it could be
import java.time.Instant;
Instant timestamp = Instant.now();
long timeLimitMillis = instant.toEpochMilli() - (15 * 60 * 1000);
if (files[i].lastModified() >= timeLimitMillis)
{
}