Android Question Knowing if DST is active for a prior or future date

[andyp]

Hi

It is simple to know if DST is active

Dim daylightsavings As Int 
        If IsDaylightInEffect = True Then
            daylightsavings = 1
            Else
                daylightsavings = 0
        End If

However, what if I wanted to know if DST would be active on some past or future date?

If there any way to find this out?

Thank you
Andrew

 

[Erel]

Where does IsDaylightInEffect come from?

You can use DateTime.GetTimeZoneOffsetAt to get the timezone at any given time.
In most cases when a developer calls the time zone related methods then it is a sign that he is doing something wrong.

 

[andyp]

Sorry....

Sub IsDaylightInEffect As Boolean
    Dim currentTimezone As Double = DateTime.TimeZoneOffset
    Dim t As Long = DateTime.Now
    For i = 0 To 11 Step 2
        If DateTime.GetTimeZoneOffsetAt(t) < currentTimezone Then Return True
        t = t + i * DateTime.TicksPerDay * 30
    Next
    Return False
End Sub

My issue is: I want to display the time of sunrise at the users location. I calculate the sunrise time based on GMT. Using the code above, I know if daylight savings is active or not, and add an hour if necessary (DST).

But what if I want to calculate the time of sunrise on some arbitrary date (at the users location). For example, how do I know if DST is active on the 1st March?

Thank you

 

[Erel]

Sub Process_Globals
   Private MinTimeZone As Double
End Sub

Sub Activity_Create(FirstTime As Boolean)
   If FirstTime Then
       FindMinTimeZone
   End If
   Log(IsDST(DateTime.Now))
   Log(IsDST(DateTime.DateParse("08/01/2000")))
End Sub

Private Sub FindMinTimeZone
   Dim t As Long = DateTime.Now
   MinTimeZone = DateTime.GetTimeZoneOffsetAt(t)
   For i = 1 To 400
       Dim m As Double = DateTime.GetTimeZoneOffsetAt(t)
       If m <> MinTimeZone Then
           MinTimeZone = Min(m, MinTimeZone)
           Exit
       End If
       t = t + DateTime.TicksPerDay
   Next
End Sub

Public Sub IsDST(t As Long) As Boolean
   Return DateTime.GetTimeZoneOffsetAt(t) > MinTimeZone
End Sub

 

[andyp]

Thank you!! I will let you know how that goes in a day or two

 

[Peter Simpson]

Another solution is this library that does everything for you. You just need to enter your current location via GPS location or FusedLocationProvider, enter the time, and read your gettimezoneoffset from you phone and you are sorted no matter where you are located in the world. Enter one one into the library and it gives you all the results that you need.

https://www.b4x.com/android/forum/threads/astro-library.15975/#content

https://www.b4x.com/android/forum/attachments/watch_time_2-png.62779/

 

[andyp]

Hi Peter - thank you - that would have saved me several hours of coding But it has been fun de-bugging and getting the data from scratch......

 

[Peter Simpson]

I've updated my last post, you should read it again

Yes I've been using that library for years with weather apps that I've created. The library is really easy to use, you will have your app up and running perfect within 10 minutes with basically a hand full of your own lines of code

Good luck with your app...

 

[andyp]

Hi Erel

Just wanted to thank you for showing me this code. Works perfectly, and I can understand the solution.

Thanks again!
Andrew

 

[Abid Hussain]

Hi,

I'm implementing the code, and think that it needs a proviso / note of caution for anyone using it.

When you enter the time, to IsDST make sure it's not midnight of the day, as DST usually switches over about 2am (certainly it does in the UK)

So I've stuck in the following code, which fixed my issues

IsDST( t + DateTime.TicksPerHour*12)

oh, and thank you Erel for the code