I am close to throwing in the towel.

Thank you for the suggestion. Several problems have been encountered. I wonder whether I understand this tool at all. If I subtract 10:00 from 08:46, the answer given is -01:14. I used #+TBLFM: $6=$4+$5;U, as follows (please forgive the formatting):

| Phenom | Date | DoW | UTC | Hrs | ChST | |
|--------+--------+-----+-------+--------+--------+---|
| ApoG | 22 | Fr | 06:44 | -10:00 | -03:16 | |

|--------+--------+-----+-------+--------+--------+---|
#+TBLFM: $6=$4+$5;U

When I add 10:00, I think the values are sensible: 21:45 + 10:00 = 31:45.

Another problem was in trying to use an inactive org timestamp. It was not straightforward to add or subtract N hours (say, 08:00).

This it a thornier problem than I had envisioned, anyway, because in locale with time zones, the conversion factor will change at some point DURING the month.

Perhaps there is a calc procedure to convert time zones that will take into account the system's knowledge of the timezones as well as changes to/from Daylight Time.

For now,

On Wed, Dec 9, 2020 at 3:40 AM Tim Cross <theophilusx@gmail.com> wrote:

Alan E. Davis <lngndvs@gmail.com> writes:

> I have been pleased to learn that I can add / subtract hours in org-table
> to shift time zones. I am making tables of lunar/solar parameters
> relevant to tides. Org-table is a convenient way to enter data in a
> tabular format that can be printed via LaTeX. So each year, for several
> time zones, I enter these times by hand.
>
> It's very, very easy, I have learned, to collect all of these times (at
> most, maybe 12 per month) for UTC, and add or subtract to generate a column
> of times for a new time zone. Very Slick!
>
> Except that when I add, for example. 09:00 to 23:33, I guess it is pretty
> obvious what is going to happen: it would be the same day, but at 32:33 !!
>
> I have tripped up on trying to test for whether the sum is greater than or
> equal to 24:00, and then doing something interesting with it. It's
> actually pretty easy to go through all the months and find the exceptions,
> and make manual changes. But, as I usually have done, I would rather spend
> a few hours coming up with some programmatic method for making this work
> automatically!
>
> The other problem is the change of the day. I suppose I could use 0, 1,
> 2... for Sunday, Monday, Tuesday... (I am living in the United States).
>
> Has someone solved this problem?
>

Org tables support formulas which use the Emacs 'calc' program. Calc has
pretty good support for adding, subtracting, multiplying and otherwise
manipulating dates and times (this is how the org clocktable works). It
should be pretty straight forward to have a column of date + time
values, a time offset representing a timezone and a 3rd column which is
the new date/time after applying the offset. Have a look at the secton
in the org manual on table formulas and the calc manual in info.

--
Tim Cross