How can I specify the latest time of day with DateTime

To get the last instant for today:

DateTime d = new DateTime(Now.Year, Now.Month, Now.Day);
d = d.AddDays(1);
d = d.AddTicks(-1);

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In response to your edit, here's what I would do:

DateTime start = new DateTime(Now.Year, Now.Month, Now.Day);
DateTime end = start.AddDays(1).AddTicks(-1);

// Or - just use end = start.AddDays(1), and use a < for comparison

If you already have a DateTime object created and want to replace the time with the 11:59:59PM for that given date, then you can use the .Date property to get the date with time set to 00:00:00 and then add the hours, minutes and seconds. For example:

var dt = yourDateInstance.Date.AddHours(23).AddMinutes(59).AddSeconds(59);

If by latest time, you mean 11:59:59 PM, then this should also work:

var dt = new DateTime(Now.Year, Now.Month, Now.Day, 23, 59, 59);

DateTime d = DateTime.Today.AddDays(1).AddTicks(-1);