Afternoon vs. evening...what time does it change?

Because I am in the business of invitations, this sort of thing matters to me, especially when I am spelling out the time on an invitation (i.e. "five o'clock in the evening" instead of 5 p.m.). The problem is, I get so many questions about when it is appropriate to use "in the evening" vs. "in the afternoon". Does evening begin at 5 pm or 6 pm?

Here is the official wikipedia definition of evening:

"Evening is the period in which the daylight is decreasing, between the late afternoon, around the time when dinner is taken. Though the term is subjective, evening is typically understood to begin before sunset, during the close of the standard business day (about 6 pm) – and extend until dusk, the beginning of night. Evening thus spans the period of twilight, but begins before it and depending on definition may extend past its end."

However, depending upon the time of year or where you work "during the close of business day and extend[ing] until dusk" could mean from 5-5:15 during the winter (and your job ends at 5 pm instead of 6 pm)...a very short evening indeed.

To make matters worse, the definition of afternoon is as follows:

"Afternoon is the time of day from 12:00 noon to -depending upon context- evening, sunset, or 18:00 [6 pm]."

Clearly, looking to definitions isn't very helpful. The best part, both definitions end with "[the specific time] is very subjective." Ugh.

Here's what I subscribe to and recommend. If your party starts at 5 pm or later, it is okay to write "five o'clock in the evening". If you party starts at 8 pm or later, you may want to say "eight o'clock in the night". If your party is before 5 pm, stick with "in the afternoon". Now, if you want to avoid it all and be less formal, just say "5:00 pm" or "8:00 pm."

Oh, one more thing. If you are spelling out the time, you need to also spell out the date and the year to be consistent. Any questions? Clear as mud...