Time issues

This issue, against the rules, reports more than one bug but they are kind of cousins.

A more distant cousin.
This one works
```
gmt basemap -R1920T/2028T/6800/7400 -JX12c/0.5c -Ba -f0T --TIME_EPOCH=0000-01-01T -png lixo
```

but this would fill your disk with a multi-GB PS file. The problem is, as **-Vd** would show, that the guessed annotations are **30 sec** apart.
```
gmt basemap -R1920T/2028T/6800/7400 -JX12c/0.5c -Ba -f0T --TIME_EPOCH=0000-01-01T --TIME_UNIT=y -png lixo
```

So, adding the relevant information **--TIME_UNIT=y** makes that `1920T` is no longer recognized as time??? Or the time unit is lost?

Now, this is what? A excess rounding?
```
echo 1921.0417 | gmtconvert -f0T --TIME_EPOCH=0000-01-01T --TIME_UNIT=y
1921-02-10T00:00:00
```

When, with GMT.jl, I save a file of that type of data, I get
```
1921-01-16T01:45:16 6986
1921-02-15T11:56:51 6946
1921-03-17T22:08:25 6879
```

but that is also wrong, as both Matlab and Julia confirm, the right date should be

```
1921-01-16T05:17:31.2 6986
1921-02-15T15:00:00.0 6946
1921-03-18T00:42:28.7 6879
...
```