const SECS_PER_DAY: int = 24 * 60 * 60
var my_date_time: int = Time.get_unix_time_from_datetime_string("2020-01-30 00:00:00")
my_date_time += 5 * SECS_PER_DAY
var new_date_time_str: String = Time.get_datetime_string_from_unix_time(my_date_time)
This is untested code, and may have errors, but that's the general approach.
I've used your method in another language (maybe Perl or PHP), but I don't think GDScript supports it.