If the parameter is a date field

function FirstDayOfMonth
    lpar tdVar
    local lcWasDateFormat, llWasCentury, lcReturn
    assert vartype(tdVar) = 'D'

    lcWasDateFormat = set('date')
    llWasCenturyOn = set('century') = 'ON'
    set century on
    set date to ymd

    lcReturn = transform(tdVar - day(tdVar) + 1, '@D')

    set date to &lcWasDateFormat
    if not llWasCenturyOn
        set century off
    endif
    return lcReturn
endfunc

>Hello at alls.
>I wonder to know with one the way to get the first month day since any date and in format DD/MM/YYYY.
>TIA