setting date-time field from string

setting date-time field from string

hello everyone, i hope someone could help me.

i have a date-time field in a form that i want to fill in from two separate fields of date, and time.
i need to combine the two fields to a one date-time field but can make it work. 
i tried to convert the values to string using toString("dd/MMM/yyyy") & toString("HH:mm:ss") and set the date-time field with the two variables by adding them together like this:
input.Date_Time = (date_str + " " + time_str).toDateTime("dd/MMM/yyyy HH:mm:ss");

i would appreciate any ideas of why it doesn't work for me or how to make it work in different ways.

thank you for the help!