getting assertion error despite outputting sometime

**Your code so far**

```
def days_later(days_added):
if days_added == 1:
return '(next day)'
elif days_added > 1:
return f'({days_added} days later)'
else:
return ''
def add_time(start, duration, day = False):
week_days = [
"monday",
"tuesday",
"wednesday",
"thursday",
"friday",
"saturday",
"sunday"
]
ndays = 0
initial_hours, initial_mins = start.split(":")
if "12" in initial_hours and "AM" in initial_mins:
initial_hours = '0'
initial_mins, initial_period = initial_mins.split(" ")
added_hours, added_minutes = duration.split(":")
initial_hours, initial_mins = int(initial_hours), int(initial_mins)
added_minutes, added_hours = int(added_minutes), int(added_hours)
new_period = initial_period
if "PM" in initial_period:
initial_hours += 12
total_minutes = initial_mins + added_minutes + ((initial_hours + added_hours) * 60)
minutes_display = round(total_minutes % 60)
minutes_left = total_minutes
ndays = int((total_minutes / 1440))
if total_minutes >= 1440:
minutes_left = total_minutes % 1440
hours_left = int(minutes_left / 60)
if minutes_left <= 720:
new_period = "AM"
else:
new_period = "PM"
hours_left = hours_left - 12
if hours_left == 0:
hours_left = 12
end_result = f'{hours_left}:{minutes_display:02} {new_period}'
if day:
day = day.lower()
selected_day = int(((week_days.index(day)) + ndays) % 7)
current_day = week_days[selected_day]
end_result += f", {current_day.capitalize()} {days_later(ndays)}"
else:
end_result = " ".join((end_result, days_later(ndays)))
return(end_result)
```

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**Challenge:** Scientific Computing with Python Projects - Time Calculator

**Link to the challenge:**