fix(GUI): automatically use the first column of ObsDF to get the dates

Refs #38
3 jobs for dev in 6 minutes and 15 seconds (queued for 1 second)
Status Name Job ID Coverage
  Check
passed check_as_cran_devel #177206

00:01:54

passed check_as_cran_oldrel #177207

00:01:32

passed check_as_cran_patched #177205

00:02:48