One of the learning curve issues when moving from a client-server environment where all the application users were in the same building to developing for the web is dealing with different time zones.
Dates have times whether you want them or not
The first time I remember being bitten by the time zone issue was a several years ago and it’s probably not what you’d expect.
My application had an Order table in a SQL Server database with an OrderDate field (DateTime data type). I wasn’t interested in the time part, so I was setting the value by using DateTime.Today, which gives today’s date with the time part set to 00:00:00 (midnight).
My test user called me to say that his OrderDate values were showing up off by 1 day. When retrieving the order, the web server was returning the order information in JSON. I didn’t realize at the time that browsers will automatically “time-zone-shift” JSON dates to the browser’s computer’s time zone. So, if the order date was “1/1/2019 00:00:00” and the test user’s time zone was 6 hours behind UTC – the browser (not realizing that the time part was not significant) translated that value to “12/31/2018 18:00:00”. My UI was formatting the date to only show the date part, so the OrderDate field value was showing as “12/31/2018” when it should have been “1/1/2019”.
This turned out to be a much trickier problem than I initially thought. This was when I realized the need for a Date data type (since there wouldn’t be any concept of time zone shifting with a Date-only type). I know SQL Server has a Date type, but C# still doesn’t.
At the time, I was in a hurry so I cheated a little bit. I started stamping the time part as noon (12:00:00) as opposed to letting it default to midnight (00:00:00). I didn’t care about the time part and it’s never displayed. This gives me 12 hours leeway in either direction with the date part being changed. Apparently some islands in the Pacific do have +13 and +14 time zones, but I was pretty sure that particular application wasn’t going to be used there.
That application has since been retired but I never found a more elegant solution to this problem. If anyone knows of one, please let me know.