date helpers

DateHelpers.cs

public class DateHelpers
{
    public static DateTime GetDayStart(DateTime date, string timezone)
    {
        var timezoneInfo = TimeZoneInfo.FindSystemTimeZoneById(timezone);
        var timezoneNow = TimeZoneInfo.ConvertTimeFromUtc(date, timezoneInfo);
        return TimeZoneInfo.ConvertTimeToUtc(timezoneNow.Date, timezoneInfo);
    }

    public static DateTime GetWeekStart(DateTime date, string timezone)
    {
        var timezoneInfo = TimeZoneInfo.FindSystemTimeZoneById(timezone);
        var timezoneNow = TimeZoneInfo.ConvertTimeFromUtc(date, timezoneInfo);
        var diff = (7 + (timezoneNow.DayOfWeek - DayOfWeek.Sunday)) % 7;
        var weekStartUtc = timezoneNow.AddDays(-1 * diff).Date;
        return TimeZoneInfo.ConvertTimeToUtc(weekStartUtc, timezoneInfo);
    }
}