# Importing the necessary libraries

from openpyxl import load_workbook

from openpyxl.styles import numbers

​

# Filename of the Excel file

filename = 'path/to/your/file.xlsx'

​

# Name of the sheet containing the dates

sheet_name = 'Sheet1'

​

# Load the workbook

workbook = load_workbook(filename)

​

# Select the desired sheet

sheet = workbook[sheet_name]

​

# Iterate through the cells in the desired range

for row in sheet.iter_rows(min_row=1, min_col=1, max_row=sheet.max_row, max_col=sheet.max_column):

    for cell in row:

        # Check if the cell value is a date or datetime

        if cell.data_type == 'd' or cell.data_type == 'n':

            cell.number_format = numbers.FORMAT_DATE_DDMMYYYY  # Change the date format to your desired format

​

# Save the modified workbook

workbook.save('path/to/your/modified_file.xlsx')

Assuming that you source date that is in the number format dd.mm.yyyy stored as an excel date serial and only formatted to display as dd.mm.yyyy then the best fix is to select the cells you want to modify. Go to your home tab, and select the number format and change it to General. See Green circles in image below. IF the format is already set to general, or when you switch it to general your numbers do not change, then it is most likely that your date in dd.mm.yyyy format is actually text. and will needed to be converted as per OPTION 2 below. However, if the number does change when you set it to general, select the arrow in the bottom right corner of the number area (see red circle).