NodaTime

NodaTimeUtil.cs

using System;
using NodaTime;

namespace Sample
{
    public class NodaDateTimeUtil
    {
        public static DateTime ConvertToUtcFromCustomTimeZone(string timezone, DateTime datetime)
        {
            var zone = DateTimeZoneProviders.Tzdb.GetZoneOrNull(timezone);
            var localtime = LocalDateTime.FromDateTime(datetime);
            var zonedtime = localtime.InZoneLeniently(zone);
            return zonedtime.ToInstant().InZone(zone).ToDateTimeUtc();
        }

        public static DateTime ConvertToCustomTimeZoneFromUtc(string timezone, DateTime utcDateTime)
        {
            var customTimeZone = DateTimeZoneProviders.Tzdb.GetZoneOrNull(timezone);
            return Instant.FromDateTimeUtc(utcDateTime)
                        .InZone(customTimeZone)
                        .ToDateTimeUnspecified();
        }

        public static DateTime ConvertToUtc(DateTime dateTime, int offsetInMinutes)
        {
            var offset = Offset.FromHours(offsetInMinutes / 60);
            var localDateTime = LocalDateTime.FromDateTime(dateTime);
            return new OffsetDateTime(localDateTime, offset).ToInstant()
                                                            .ToDateTimeUtc();
        }

        public static DateTime ConvertFromUtc(DateTime dateTime, int offsetInMinutes)
        {
            var offset = Offset.FromHours(offsetInMinutes / 60);
            var instant = Instant.FromDateTimeUtc(dateTime);
            return instant.WithOffset(offset)
                        .LocalDateTime
                        .ToDateTimeUnspecified();
        }

        public static int DiffDate(DateTime dtStart, DateTime dtEnd)
        {
            LocalDate ldtStart = LocalDate.FromDateTime(dtStart.Date);
            LocalDate ldtEnd = LocalDate.FromDateTime(dtEnd.Date);
            Period period = Period.Between(ldtStart, ldtEnd, PeriodUnits.Days);
            return period.Days;
        }
    }
}