I don't know, if it works:
columnX.ControlSource='ttod(table.DateTimeField)'
columnX.AddObject('control2','ShowOnlyDate')

class ShowOnlyDate based on TextBox:
Init()
this.ControlSource=''
GotFocus()
this.value=ttod(evaluate(table.DateTimeField))
LostFocus() or Valid()
replace (table.DateTimeField) with dtot(this.value)
TableUpdate() ...

and with Grid.AfterRowColChange() or ColumnX.DynamicCurrentControl
set Control to 'text1' or 'control2'

Pavel