// sample value
// any date in valid date format:
// d.m.y, m/d/y, m-d-y, y.m.d, y/m/d, y-m-d  
Set V[mydate] "01.12.2019"

// check that date is in the past
Set V[diff] "&V[today_d.m.y]" - "&V[mydate]"
if V[diff<0]
  Message "E: &V[mydate] is not in the past" -statusline  
endif

Components
InputAssistant