The below code would give you the number of days since the file was last modified. This is using filpath so it's assuming that the file is in a volume defined in x3, in this case in the TMP folder.

(date$ - [01/01/1970]) - (filinfo(filpath("TMP", ",myfile", "txt") , 9) / 86400)

The below would round it

fix( (date$ - [01/01/1970]) - (filinfo(filpath("TMP", ",myfile", "txt") , 9) / 86400) )

You can make use of the VOLUME class to get the information you need, remember that the area you are the file is located needs to be defined as a "Sage X3 storage area" and/or the is must be defined in the runtime sandbox configuration file. The below will return the date the file created as a datetime value. 

local Integer XASTATUS
local Datetime XDATETIME
local Char XVOLUME
local Char XFILENAME
[L]XVOLUME = "TMP" # X3 Storage area name
[L]XFILENAME = "myfile.txt" # File name + extension

[L]XASTATUS = Func ASYRSTO.GETFILEDATETIME([L]XVOLUME, [L]XFILENAME, [L]XDATETIME)