Alan E. Davis <lngn...@gmail.com> writes:
> 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. > What did you expect for 8:46 - 10:00? Looks correct to me or were you expecting 22:46 (24:00 - 01:14)? This would mean 21:45 + 10:00 should be 07:45. I think when your working with times like this, you need to include the date to help make sense of the result. > Another problem was in trying to use an inactive org timestamp. It was not > straightforward to add or subtract N hours (say, 08:00). > You probably need to use the ort-timestamp-to-time and org-timestmap-from-time to convert the timestamp to a 'time' value (I suspect it uses either ms or sec since epoch as the base). Convert to time, add/subtract offset, convert back to inactive timestamp. > 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, > The big pain with working on time and timezones is the daylight savings complication. This is really tricky because the start and end date tend to be influenced by politics (I've seen DST change because of some event, like Olympic games or to coincide with easter holiday etc) and some states/geographies may decide not to use DST while others do (for example, in Australia, some states have DST and some don't - so for half the year, all the eastern states have the same timezone, but then for half the year, 3 are the same and one is different). There is some information in the calendar section of the emacs manual which might be useful and it does have a section on working with DST (I've not read it). In addition to the org mode functions to manipulate dates and times, there are also various elisp functions you can also use. It is a thorny problem because of the edge cases, but the basic functions are all there. Your best bet is to probably write a function which accepts a full date+time and UTC offset in minutes which returns a new date+time value and then call that function in your table formula. -- Tim Cross
