Date-time dialog: seconds change when changing hours or minutes

[Peter Mousley]

When changing the hour or minute in the date-time dialog, the seconds value changes (it typically decrements by 1). Thanks to Pep i Mercè Mas la Roca for finding and reporting this bug.

I've traced the bug back to the calculation of hours/minutes/seconds from the Julian date - there is a floating-point truncation error. For example, a Julian date converted to seconds might be calculated as 9.9999996, which is truncated to 9 instead of rounded to 10. I'll propose a merge that fixes it.

Peter