.net - Getting number of seconds in a day -

i have list of datetimes have hold time midnight, , number of seconds website considered "up" on day.

i need work out percentage of "uptime", need know how many seconds in day. wondering if there in .net similar datetime.daysinmonth(year, month) seconds in day.

my google-fu seems have let me down, , can find .net constant number of seconds in day? - doesn't take account daylight savings.

i think simple

var secondsinday = (startday.adddays(1) - startday).totalseconds; 

but wondering if correct (simple not exhaustive tests indicate is)?
there more efficient way of calculating number of seconds in given day?

please note - needs take account of daylight saving hours

here function asked:

public int secondsinday(datetime datetime, string timezoneid) {     var dt1 = datetime.date;     var dt2 = dt1.adddays(1);     var zone = timezoneinfo.findsystemtimezonebyid(timezoneid);      var dto1 = new datetimeoffset(dt1, zone.getutcoffset(dt1));     var dto2 = new datetimeoffset(dt2, zone.getutcoffset(dt2));     var seconds = (dto2 - dto1).totalseconds;      return (int) seconds; } 

as example:

int s = secondsinday(new datetime(2013, 1, 1), "central standard time"); // 86400  int s = secondsinday(new datetime(2013, 3, 10), "central standard time"); // 82800  int s = secondsinday(new datetime(2013, 11, 3), "central standard time"); // 90000