Code:
/* zero out the unwanted characters */
for (i=18; i<SIZE; i++) {
croppedString[i] = '\0';
}
croppedTime = RemoveColon (croppedTime, mytime);
int convertedNum = atoi(croppedTime);
I don't know why you need any of this.
hh:mm:ss should be stored in mytime, as a direct result of the sscanf call.