-B+e (to skip first or last annotation) not working with dates

Hi there, never 2 without 3 (not sure this translate well in English)... anyway,

I'm trying to finish [a code](https://forum.generic-mapping-tools.org/t/plot-w-flag-bug/2556/6?u=planetgus) and I'm almost done apart from the first x annotation that bothers me.

I found in the doc the `-B+e` option, and ... it didn't work. After some trial the problem seems to be only for "month" annotation.

MWE :
```
gmt basemap -R2000-01-01T/2001-01-01T/-1/1 -JX5c -Bafg+e --FORMAT_DATE_MAP=o --FORMAT_TIME_PRIMARY_MAP=a -png test
```

Start by running the provided `gmt basemap` MWE and inspect how `-B+e` handles month annotations with the date format settings shown. Trace the date-axis annotation path to determine why the first x annotation is not skipped. Done means the first or last month annotation is omitted as requested while other date annotations remain correct.

Written by the indexing model from the issue text.