UNIX timestamp from date + time

Yes the time dialogue returns a millisecond value using today's date, the easiest thing to do is convert the time to millisecond (h*60 + m) * 60000 then using the date function this gives the time/date for midnight add the two and you have a correct representation of the input time and date, if you want to correct for time zone add in the time zone offset from the datetime function.

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utc = d + t - DateTime.DateParse(DateTime.Date(DateTime.Now)) - (DateTime.TimeZoneOffset * DateTime.TicksPerHour)

is it correct ?

Almost, I would use GetTimeZoneOffsetAt not TimeZoneOffset as 'TimeZoneOffset' gets the offset for the current date but 'GetTimeZoneOffsetAt' gets the offset for the specified date taking DST (summer/winter time) into account but then I use this for creating calendar events which could be in a different summer/winter time, also the offset is added not subtracted. Play around with it to see how it works out.

Thanks, lagore !
If not to forget to make + ( DateTime.GetTimeZoneOffsetAt(DateTime.DateParse(input_date)) * DateTime.TicksPerHour) - seems, at last it's correct

utc_datetime = d + t - DateTime.DateParse(DateTime.Date(DateTime.Now)) + (DateTime.GetTimeZoneOffsetAt(d) * DateTime.TicksPerHour)
where d, t - Date, Time from dialogs.