robust sorting of dates

[andrew-MET]

Thanks for the quick PR.

I moved the sorting to inside the !is.null to make sure the app doesn't crash when it has no data to fill the select.

Also there are some styling considerations to help readability I should have told you about:

• <-, = and mathematical operators should have one space on each side. I tend to think of them as words, which helps to remind me
• If <- or = appear on consecutive lines they should be underneath each other
• All commas should be followed by a space
• We try to keep the lines shorter than 80 characters long, though this still needs fixing in many places

In general we try to follow the tidyverse style guide

Also, dplyr / tidyr column selections need to be preceded by the .data pronoun, i.e. .data$colname. This prevents errors when running R CMD CHECK on the package.

Otherwise I think this fixes #26

[leneoMET]

Thanks for the corrections! I should have seen that the sorting should be inside !is.null. I will look into the tidyverse style guide for future commits.