Android Question Unparseable GMT date

marcick · Mar 30, 2025

Hello everyone,

I work with dates from a GPS module, which are in GMT (Greenwich Mean Time). I then convert them by adding the time zone and daylight saving time to display the Italian time.
With the code I use, everything works fine except when the GMT time falls exactly at the transition to daylight saving time.
How do you suggest handling this better?

B4X:

    DateTime.DateFormat = "yyyy-MM-dd HH:mm:ss"
    Dim dl As Long=DateTime.DateParse("2025-01-30 01:20:31")       
    dl=dl+(DateTime.GetTimeZoneOffsetAt(dl)*3600000)
    Log(DateTime.Date(dl)) --->>> 2025-01-30 02:20:31, correct
    
    Dim dl As Long=DateTime.DateParse("2025-03-30 01:20:31")
    dl=dl+(DateTime.GetTimeZoneOffsetAt(dl)*3600000)
    Log(DateTime.Date(dl)) --->>>2025-03-30 03:20:31, correct
    
    Dim dl As Long=DateTime.DateParse("2025-03-30 02:20:31")
    dl=dl+(DateTime.GetTimeZoneOffsetAt(dl)*3600000)
    Log(DateTime.Date(dl))  --->>> java.text.ParseException: Unparseable date: "2025-03-30 02:20:31"

Erel · Mar 30, 2025

Never add offsets to time instances like this. It will not work as you expect.
You should instead add the time zone to the string:

B4X:

DateTime.DateFormat = "yyyy-MM-dd HH:mm:ssZ"
Dim dl As Long = DateTime.DateParse("2025-01-30 01:20:31" & "+0000")
Log(DateTime.Date(dl))

MicroDrie · Mar 30, 2025

What about this?

Time offset:

    Dim TimeOffset As Double = DateTime.GetTimeZoneOffsetAt(DateTime.Now) * 60 * 60 * 1000    ' hours ticks
    DateTime.DateFormat = "dd-MM-yyyy HH:mm:ss"
    Dim dl As Long = DateTime.Now
    Log($"
Time offset: ${TimeOffset} hours.
Seconds ticks: ${dl}
${DateTime.Date(dl)}
Offset seconds ticks: ${dl-TimeOffset}
${DateTime.Date(dl-TimeOffset)}"$)
End Sub

Which give as result:

Time offset: 7200000.0 hours.
Seconds ticks: 1743344815478
30-03-2025 16:26:55
Offset seconds ticks: 1.743337615478E12
30-03-2025 14:26:55

emexes · Monday at 5:31 AM

Minor point, but important for some use-cases:

GPS time is not quite GMT time - it is offset by the number of leap seconds that have occurred since GPS began (I think) eg:

http://www.leapsecond.com/java/gpsclock.htm

Erel · Monday at 6:08 AM

MicroDrie said:
What about this?

Same as before: Never add offsets to time instances like this. It will not work as you expect.

marcick · Monday at 7:08 AM

Erel said:

B4X:

DateTime.DateFormat = "yyyy-MM-dd HH:mm:ssZ"
Dim dl As Long = DateTime.DateParse("2025-01-30 01:20:31" & "+0000")
Log(DateTime.Date(dl))

Looks like incorrect:

DateTime.DateParse("2025-01-30 01:20:31" & "+0000") -->> 2025-01-30 02:20:31+0100 , correct
DateTime.DateParse("2025-03-30 01:20:31" & "+0000") -->> 2025-03-30 03:20:31+0200 , WRONG, should be still 02:20:31
DateTime.DateParse("2025-03-30 02:20:31" & "+0000") -->> 2025-03-30 04:20:31+0200 , correct

Erel · Monday at 12:41 PM

What is your timezone?

marcick · Monday at 12:44 PM

Italy, normally +1, become +2 with daylight

..... I think I'm il tilt. Should be ok as above, sorry

MicroDrie · Monday at 12:50 PM

Erel said:
Same as before: Never add offsets to time instances like this. It will not work as you expect.

I don't want to be stubborn, but I don't understand why the given offset calculation would be wrong. The core library import java.util.SimpleTimeZone library. Oracle says:

SimpleTimeZone is a concrete subclass of TimeZone that represents a time zone for use with a Gregorian calendar. The class holds an offset from GMT, called raw offset, and start and end rules for a daylight saving time schedule. Since it only holds single values for each, it cannot handle historical changes in the offset from GMT and the daylight saving schedule, except that the setStartYear method can specify the year when the daylight saving time schedule starts in effect.

The program asks SimpleTimeZone library in the core lib for the offset in hours from the given local time. Then the given time is converted to the number of ticks. From this, the number of difference hours ticks is calculated and subtracted. Then the result of remaining ticks is converted to GMT time.

If we limit ourselves to the calculation method and taking into account the absence of the exceptions mentioned by Oracle, I do not see what would be wrong with calculating the time difference between two time zones.

I learned during my time maintaining laboratory instruments that a result must be verified with a second source, where the outcome of that second measurement/calculation is based on a different measurement method.

A second source is timezones with the same result as the program:

Edit: P.S. Perhaps I should have provided some background information on the program to clarify it.

Erel · Monday at 1:10 PM

Italy timezone: https://www.timeanddate.com/time/change/italy

On 30/3 at 01:00 GMT the timezone changed to GMT + 2
So 30/3 01:21 GMT => 30/3 03:21 GMT + 2

MicroDrie said:
I don't want to be stubborn, but I don't understand why the given offset calculation would be wrong.

DST timezones are very confusing. As a general rule - simple arithmetic do not work with date and time.

BTW, the code in the first post shows one such edge:

B4X:

 Dim dl As Long=DateTime.DateParse("2025-03-30 02:20:31") 'throws exception

In the developer timezone this is not a valid time instance. No clock in Italy will show this time. There are also other strings that represent two different time instances.

Android Question Unparseable GMT date

marcick

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