Exercise 1 solution (Python)

def to_min(t):
    h, m = t.split(":")
    return int(h) * 60 + int(m)

def to_clock(mins):
    return f"{mins // 60:02d}:{mins % 60:02d}"

def turn_back_time(dark, remaining_min, margin=30):
    return to_clock(to_min(dark) - remaining_min - margin)

print(turn_back_time("16:10", 140))
print(turn_back_time("16:10", 140, 60))

Output:
13:20
12:50

Dark at 16:10 is 970 minutes after midnight. Subtract the 140 minutes needed to get back to the road and the 30 minute margin and you get 800 minutes, which is 13:20. A 60 minute margin moves the turn-back time half an hour earlier, to 12:50. The turn-back time is worked out before you start, from the time it gets dark, not from how you feel when you are tired.