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How to get a valid daylight saving time - Part 2

Intr-un post anterior am povestim putin despre TimeZone si daily light saving. Prin metoda "IsDaylightSavingTime" a unui TimeZoneInfo spuneam ca se poate verifica daca pentru o anumita data avem ora de vara sau de iarna.
Lucrurile se complica putin cand nu cunoastem TimeZoneInfo pe care il are clientul. In acest caz, pe baza orei trebuie sa identificam pe ce time zone se afla clientul. Acest lucru nu se poate determina in totalitate, dar putem identifica un time zone care partial asemanator.
Un obiect de tip TipZoneInfo contine metoda "GetAdjustmentRules" care ne returneaza o lista de reguli care se folosesc pentru a ajusta ora (de exemplu ora de vara sau de iarna). Fiecare regula defineste perioda de inceput si de sfarsit dar din pacate perioada nu este definita la nivel de zi ci doar de luna. De exemplu ni se specifica ca in luna martie se termina perioada in care se aplica regula, dar nu se specifica data exacta cand aceasta se termina.
A doua problema apare in cazul in care aplicatia ruleaza pe un server care are setat time zone-ul pe "Coordinated Universal Time". Pentru acest time zone nu exista ora de vara sau de iarna, din aceasta cauza incercati sa verificati daca ora data este de vara sau de iarna folosind metoda "IsDaylightSavingTime" o sa va trezici cu rezultate ciudate.
In unele cazuri nu avem altceva de facut decat sa ne vazam de ora pe care clientul o trimite si pe baza careia sa obtinem un time zone. Acesta solutie trebuie aplicata doar daca este ultima solutie, deoarece niciodata nu trebuie sa ne bazam pe ora clientului si sa o comparam cu cea a serverului, dar in unele cazuri aceasta este singura optiune.
Pentru a obtine ultima duminica dintr-o luna va recomand sa folositi urmatoarea metoda:
public static DateTime LastDay(this DateTime current, DayOfWeek dayOfWeek)
{
DateTime lastDay = current.Last();
return lastDay.AddDays(Math.Abs(dayOfWeek - last.DayOfWeek)*-1);
}
Dupa ce comparam ora clientului cu cea a serverului trebuie sa verificam daca data curenta este cu ora de vara sau de iarna. Pentru acest lucru putem sa ne folosim de metoda data mai sus si sa verificam data data este cuprinsa intre ultima duminica din luna Octombrie si ultima duminica din luna Martie.
Acesta nu este cel mai sigur mecanism, dar in unele cazuri poate sa fie singura solutie pentru a putea determina in ce time zone se afla clientul.

Comments

  1. Ciudat.. Cum fiecare tara (culture in .NET terms) poate avea propriile reguli legate de ora de vara, si unele tari sunt in mai multe timezones (GMT+n), nu prea vad cum ai putea afla daca o data&ora e in summer time sau nu fara a sti macar si culture-ul din care provine.. (multe tari de la ecuator sau Rusia nu folosesc DST, evident, chiar daca sunt in GMT+2 de ex).

    Sa nu mai vorbim de ciudateniile din USA, unde state din acelasi timezone folosesc sau nu DST dupa cum cred de cuviinta.. :)

    ReplyDelete

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