Copy the date in org table - emacs

Question

Suppose such a spreadsheet in org table
|------------+-------+------------+--------+--------+------------|
| Date | Items | Unit Price | Amount | Amount | Categories |
|------------+-------+------------+--------+--------+------------|
| 2019/09/17 | A | 2.64 | 1 | 2.64 | materials |
| | B | 52.67 | 2 | 105.34 | diagnosis |
| | C | 3.08 | 1 | 3.08 | materials |
| | D | 3.85 | 2 | 7.7 | materials |
| | E | 33.66 | 2 | 67.32 | materials |
| | F | 40 | 1 | 40 | treatments |
| | G | 16.5 | 1 | 16.5 | materials |
| | H | 4 | 3 | 12 | treatments |
| | I | 40 | 1 | 40 | bed |
| | M | 6 | 13 | 78 | treatments |
|------------+-------+------------+--------+--------+------------|
#+TBLFM: $5=$3*$4
How could copy the date 2019/09.17 to the bottom of data column?

Answer 1

The link that #manandearth posted in the comments describes how to duplicate (perhaps with slight modifications) the entries in a column. Briefly, pressing S-RET in a cell duplicates its contents from the cell above (if it is not empty) - if the cell is full and the next cell is empty then it duplicates the full cell to the empty cell. If the contents are numeric, then the "duplication" involves a slight modification: it increases the value by 1. The same happens with a date: it increases the date to next day (but the date has to be in a format that Org mode recognizes: either an active date <YYYY-MM-DD> or an inactive data [YYYY-MM-DD]). The increment by default is 1 in these cases, but can be set to something else by setting the variable org-table-copy-increment to a different value. That's the "interactive" case I mention in my comment.
The other way to fill a column in a table is by using a formula. For example here's a formula to fill the first column with a copy of the first entry in the column:
#+TBLFM: #3$1..#>$1 = #2$1
This says: Set all rows from row 3 (#3) to the last row (#>) of column 1 ($1) to the value of the cell in row 2 (#2), column 1 ($1). Note that row 1 is the header. Press C-c C-c on the table formula line above and ... wait, what happened?
|------------+-------+------------+--------+--------+------------|
| Date | Items | Unit Price | Amount | Amount | Categories |
|------------+-------+------------+--------+--------+------------|
| 2019/09/17 | A | 2.64 | 1 | 2.64 | materials |
| 13.196078 | B | 52.67 | 2 | 105.34 | diagnosis |
| 13.196078 | C | 3.08 | 1 | 3.08 | materials |
| 13.196078 | D | 3.85 | 2 | 7.7 | materials |
| 13.196078 | E | 33.66 | 2 | 67.32 | materials |
| 13.196078 | F | 40 | 1 | 40 | treatments |
| 13.196078 | G | 16.5 | 1 | 16.5 | materials |
| 13.196078 | H | 4 | 3 | 12 | treatments |
| 13.196078 | I | 40 | 1 | 40 | bed |
| 13.196078 | M | 6 | 13 | 78 | treatments |
|------------+-------+------------+--------+--------+------------|
#+TBLFM: #3$1..#>$1 = #2$1
It does not quite work in this case for a technical reason: Org mode uses Calc in table formula calculations and Calc looks at 2019/09/17 and says: "Aha, I have to divide 2019 by 9 and then divide the result by 17", and fills the rest of the column with the result of the divisions: 13.196078. You may have meant 2019/09/17 to be a date, but Org mode does not know that: it gives it to Calc which interprets it as an arithmetic expression. The solution here is the same as in the linked answer: make Org mode aware that it's a date by making it either an active date: <2019-09-17> or an inactive date: [2019-09-17]:
|------------------+-------+------------+--------+--------+------------|
| Date | Items | Unit Price | Amount | Amount | Categories |
|------------------+-------+------------+--------+--------+------------|
| [2019-09-17] | A | 2.64 | 1 | 2.64 | materials |
| [2019-09-17 Tue] | B | 52.67 | 2 | 105.34 | diagnosis |
| [2019-09-17 Tue] | C | 3.08 | 1 | 3.08 | materials |
| [2019-09-17 Tue] | D | 3.85 | 2 | 7.7 | materials |
| [2019-09-17 Tue] | E | 33.66 | 2 | 67.32 | materials |
| [2019-09-17 Tue] | F | 40 | 1 | 40 | treatments |
| [2019-09-17 Tue] | G | 16.5 | 1 | 16.5 | materials |
| [2019-09-17 Tue] | H | 4 | 3 | 12 | treatments |
| [2019-09-17 Tue] | I | 40 | 1 | 40 | bed |
| [2019-09-17 Tue] | M | 6 | 13 | 78 | treatments |
|------------------+-------+------------+--------+--------+------------|
#+TBLFM: #3$1..#>$1 = #2$1
This does not do automatic incrementation but if that's what you want, it's easy to accomplish: Calc can do calculations on dates, so we can increment daily by adding to the date in each row the row number minus 2 (e.g. row 3 would get an increment of 3 - 2 = 1, row 4 would get 4 - 2 = 2, etc). To accomplish this, you have to get the row number of the current row: the idiom is ##. Then the formula becomes:
#+TBLFM: #3$1..#>$1 = #2$1 + ## - 2
and the table becomes:
|------------------+-------+------------+--------+--------+------------|
| Date | Items | Unit Price | Amount | Amount | Categories |
|------------------+-------+------------+--------+--------+------------|
| [2019-09-17] | A | 2.64 | 1 | 2.64 | materials |
| [2019-09-18 Wed] | B | 52.67 | 2 | 105.34 | diagnosis |
| [2019-09-19 Thu] | C | 3.08 | 1 | 3.08 | materials |
| [2019-09-20 Fri] | D | 3.85 | 2 | 7.7 | materials |
| [2019-09-21 Sat] | E | 33.66 | 2 | 67.32 | materials |
| [2019-09-22 Sun] | F | 40 | 1 | 40 | treatments |
| [2019-09-23 Mon] | G | 16.5 | 1 | 16.5 | materials |
| [2019-09-24 Tue] | H | 4 | 3 | 12 | treatments |
| [2019-09-25 Wed] | I | 40 | 1 | 40 | bed |
| [2019-09-26 Thu] | M | 6 | 13 | 78 | treatments |
|------------------+-------+------------+--------+--------+------------|
#+TBLFM: #3$1..#>$1 = #2$1+ ## - 2
The various anomalies of the display of dates (do we include the day of the week? do we include the time?) might be worked around using org-time-stamp-custom-formats but that gets us into waters that I have not explored.