The Advanced Date Time Calculator is easy to use. Also, I checked out 3 of the alternatives mentioned above: https://www.timeanddate.com/date/dateadd.html,
http://www.calculator.net/date-calculator.html and
datewheel. I prefer Advanced Date Time.

Downloaded and installed with no problems. I checked the time difference between home and London England in early March and late March and saw the difference in the daylight savings time which is very helpful for me. I installed to my laptop so I can take it to the office and set the worldclocks. This has been difficult for me since different countries use different daylight savings times. This makes adjusting the clocks much easier!

These days, more and more of my Giveawayofthday installations fail with the message: "Failed to launch the program". For example, today, the Advanced Date/Time program and the Driver update program both failed to install.
One of the vendors has charged that this is due to Giveaway's use of the Themida software wrapper, and in that particular case, the installation problem was solved by going to the vendor's website instead of the Giveawayofthe day wrapper. Unfortunately, this is not always feasible, or would be too much trouble.
Unless Giveawayoftheday can solve this problem, it is going to become a dead duck for me; not worth the time and effort.
I suspect that something that I've downloaded previously from GOTD or BitsduJour, or Shareware (and there are many of them) i.e., a PC protection program of some kind, is causing this problem, but how to know which one?
It is not feasible to find or turn off all the protection programs that may or may not be running in the background.
It seems that one Giveaway may be fighting the installation of another, in this case, most of the GOTD offerings.
Any advice, or comments from GOTD would be appreciated.

Here we go again.........went thru all the "hoops"........3 times........but get "input correct license key" error.
Several times lately get the same . I would presume cut and paste is a "no-no"
Oh well have another "brand" but thought would TRY this one......Thank goodness for REVO.

Later:
Problem fixed by reinstalling the Time Zones registry key. Sorry about the complaint, prog works fine!
win XP sp3

I have used the Time And Date site. It is very comprehensive and well done. But for the purpose of determining the time between two dates, or the date a certain time period is from another date... Which is what I almost always need... The Date Wheel site I linked to above can't be beat. Just change one value and everything else changes instantly. No choosing from drop downs, etc. A really phenomenal tool, and the "business days" function is unique as far as I know.

As for offline capability, if I am at my computer doing this sort of work, I am online. If I had iPhone I would likely also use their app (hope they will do an Android version)

If you miss this, here are web based calculators:

https://www.timeanddate.com/date/dateadd.html

http://www.calculator.net/date-calculator.html

I try to find the difference between two dates and hours but no matter which dates and hours I choose it gives the same error message: "The [From/To] time is out of range." It also start with an error message: "The [Result] is out of range."

Thank you very much for the nice program, but I have a problem:
Only 11 time zones to select from are showing in the 4 time boxes, also in the large 'All time zones' box below, the date and time show, but the location names for them are blank.
Please help.
Eldad

As a professional Project Manager, date calculations are a huge part of my job and I have to do them often. I wish TriSun Software had included screenshots or similar to tempt me to download. A great date calculator could be quite useful, but I'd need to be swayed that it is better than my current tools.

Project Managers have forever used a date wheel for calculating dates, and they do work. I have to admit that I don't like them, though. I carry one for when I am in meetings and not at my computer and need to do date calcs.

When I am at my computer, however, there is an amazing and simple-to-use online tool called "DateWheel" at http://datewheel.net/desktop/en/index.html. They say they also have an iPhone app, but I'm an android user so I can't comment on that. But this online tool couldn't be simpler... it has inputs for two dates (beginning and end) and a number for measure of time (which can be in days, weeks, months or Business Days (!)). You just change any of those parameters and the rest changes automatically. I've been using this online tool for several months now and feel I can't live without it.

Anyway, TriSun might have swayed me to download and try their product if they had shown me its interface

Free and better...

https://www.timeanddate.com/