An Explanation on Equinoxes

timeanddate.com explains in more detail as to why day and night are not exactly of equal length during the March and September equinoxes.

Why Equinoxes Do Not Have Exactly 12 Hours of Daylight

The geometric center of the sun's disk crosses the equator during the equinox, and this point is above the horizon for at least 12 hours on earth. However, the sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. These are the moments of first and last direct sunlight. The disk’s center is below the horizon during this time.

Furthermore, atmospheric refraction causes the sun's disk to appear higher in the sky than it would if earth had no atmosphere. The disk's upper edge is visible in the morning for several minutes before the disk's geometric edge reaches the horizon. Similarly, the disk’s upper edge disappears in the evening several minutes after the geometric disk passes below the horizon.

Sunrise and sunset times are calculated for the normal atmospheric refraction of 34' (or 34 arc minutes) and semi-diameter, which is the distance from the center to the edge of the sun’s disk, of 16' (arc minutes). This gives a total time of 50' (arc minutes). Therefore, the sun’s geometric center is actually 50 minutes of arc below a regular and unobstructed horizon for an observer on the surface of the earth in a level region. The sun takes more than three minutes to make up for the 50' (arc minutes) and the total lengthening due to this effect is around 6.5 minutes or more.

For observers within about two degrees of the equator, the period from sunrise to sunset is longer than the night. The date of equal day and night occurs before the March equinox at higher latitudes in the northern hemisphere. Day time continues to be longer than night time until after the September equinox. In the southern hemisphere, the dates of equal day and night occur before the September equinox and after the March equinox. There may be no regular date for when night and day are exactly equal because the day length can change by many minutes from one day to another, especially at higher latitudes.

Listed below are timeanddate.com's calculations on the dates of "equal" day and night, which apply for longitude 0, 2009:

These figures are approximate and may differ based on longitude and time zone. timeanddate.com’s Sunrise and Sunset Calculator helps you find calculations for exact dates in different locations regarding approximate equal day length.

Note: An arc minute refers to a unit for measuring small angles, used in geometry, surveying, map-making, and astronomy. An arc minute is one-sixtieth of a degree and is divided into 60 arc seconds. Small distances in the sky, as between two close stars or the apparent width of a planet's disc, are expressed in minutes and seconds of arc.

Also, any reference to “equal day and night” refers to the approximate equal amount of sunlight during the day as there is darkness in the night.