Updating date

CSV files are added manually to a folder conectivity method. The reading date aka modified date aka last updated date will be crucial for my analysis. All the daily file will be appended to a large file/ or main file, so my problem is how do I stamp the date of the file to a column and call it reading date. Please see the example below.

I really appreciate any help you can provide. 

Thanks

Iqbal

Here is an example;

Day 1 file - 4/23

Revenue date | Rooms Sold |  Avg Rate |  Revenue| Reading date |

4/23/2020            100               $100         $10000    4/23/2020

4/24/2020            120               $100         $12000    4/23/2020

4/25/2020            110               $110         $12100    4/23/2020

-------------------------------------------------------------------

12/31/2020          60                 $200         12000      4/23/2020

Here is an example;

Day 2 file - 4/24

Revenue date | Rooms Sold |  Avg Rate |  Revenue| Reading date |

4/23/2020            110               $100         $11000    4/24/2020

4/24/2020            130               $100         $13000    4/24/2020

4/25/2020            120               $110         $13200    4/24/2020

-------------------------------------------------------------------

12/31/2020          70                 $200         14000      4/24/2020

Here is an example;

Day 3 file - 4/25

Revenue date | Rooms Sold |  Avg Rate |  Revenue| Reading date |

4/24/2020            130               $100         $13000    4/25/2020

4/25/2020            130               $100         $13000    4/25/2020

4/26/2020            120               $110         $13200    4/25/2020

-------------------------------------------------------------------

12/31/2020          70                 $200         14000      4/25/2020