Python- how do I write user input to an Excel file?

You could use openpyxl to write to the workbook. Here's some basic usage, and should help avoid overwriting:

import openpyxl
wb = openpyxl.load_workbook('C:/test.xlsx')
ws = wb.active
i = 0
cell_val = ''
# Finds which row is blank first
while cell_val != '':
    cell_val = ws['A' + i].value
    i += 1
# Modify Sheet, Starting With Row i
wb.save('C:/test.xlsx')

Hope This Helps.

Edited, getting input and time:

For getting information from the user, use

x = input('Prompt: ')

However, if you want the actual current, I suggest using the time module:

>>> from time import strftime
>>> date = strftime('%m-%d-%y')
>>> time = strftime('%I:%M%p')
>>> print(date)
08-28-15
>>> print(time)
01:57AM