Timezone differences obviously enter into the correct calculation of seconds elapsed between two dates & times. At present, these seconds calculators can deal with timezones of whole hour increments. 30-minute or 15-minute timezone offsets are not currently supported.
I have observed that the default javascript timezone setting on many computers is unreliable. Though the system clock may be set more or less accurately to reflect the correct time, the timezone is often incorrect. I have included server-side logic to attempt to correct for this, but, in case of exceptions, you may have to correctly set the timezone under the "System time check" heading.
Having done that, the timezone will automatically self-select in the 7 forms. In calculation forms 1, 2, 5, 6, and 7, you may override this auto-selected setting.
Daylight Saving
Within the 4 mainland U.S. timezones, these routines also automatically adjust for Daylight Saving vs. Standard times. There are two places where such automatic adjustments kick in: First, when converting the date and time as entered into forms 1, 2, 5, 6, and 7 ('pre-calculation'), and then, in calculations 3, 4, 5, and 6, where the calculation window must generate a date ('post-submit') from a seconds computation, and display it according to the correct daylight / standard time rules.
First, on the pre-calculation side, I have included my own logic as follows: If you do not override the auto-selected timezone, the date you fill in will be used to possibly change the timezone automatically from daylight to standard time or vice-versa. If you manually select one of the 4 named U.S. timezones, that overide selection is always accepted, but you may see a warning message if you select daylight time for a standard time date, and vice-versa. If you select any of the generic UTC timezones, that override selection is always accepted, with no warning.
As of release 1.1.000, and as noted in the Browser / javascript notes, the timezone adjustment in the post-submit calculations 3, 4, 5, and 6 are now browser-independent, relying upon my own built-in logic as in the pre-calculation side.
The current (2008) U.S. laws are projected in these calculations as applying to all dates in the future. Dates prior to the modern era are treated as standard time.
Most of the above discussion applies only to the 4 named mainland U.S. timezones. I don't even attempt to handle daylight saving or other time schemes in other parts of the world. You may always use the generic UTC timezone designations as an override selection.
The following is a synopsis of Daylight Saving time as implemented in the U.S. This table is a simplified version of rules that were often applied in a haphazard or non-uniform way. But it should suffice as an explanation, and as a basis for doing the calculations more or less automatically and accurately. If you know the automatic logic to be incorrect in some specific case, you may override or adjust accordingly. If you do not want automatic daylight saving adjustments, select the corresponding UTC timezone code instead of the named U.S. timezone.
Years |
Rules for Daylight Saving time |
|---|---|
1918-1919 |
March 31 until October 31 of both years (??) |
1942-1945 |
Feb. 9, 1942 until Sept. 30, 1945 (continuous) |
1966-1985 (except for next entry) |
Last Sunday in April until last Sunday in October (2:00 am time change) |
1974-1975 |
Jan. 6, 1974 until Apr. 27, 1975 (continuous) |
1986-2006 |
First Sunday in April until last Sunday in October (2:00 am time change) |
2007... |
Second Sunday in March until first Sunday in November (2:00 am time change) |