I am writing some data to a .txt file. I am using Open[] to create the text file in the directory I want and then Write[] and WriteString[]. (I am using FileNameJoin[] to create the actual filenames). I tried this out using a test file, called test.txt (and stored in the appropriate directory), it opens, writes to, and closes the file how I want. However, every time the notebook is run, I want to create a new output .txt file, so the files are named based on the date and time (in 24 hours), so part of the filename looks like HH:MM:SS. So, I am running something that looks like:

filename = 
  FileNameJoin[{"C:", "Users", "Me", "Dir1", "Dir2", 

Mathematica gives this error:

OpenWrite::noopen: Cannot open C:\Users\Me\Dir1\Dir2\2019-07-0810:59:01.txt.

I think what it doesn't like is the colons, since if I replace : with ; it opens/creates the file just fine (and it can open a file with just the date as the name). However, this is kind of ugly, and I would really prefer to use : instead.

Any suggestions are appreciated! Thanks you guys!

  • 2
    $\begingroup$ Personally, I solve this problem be replacing XX : YY : ZZ by XXhYYmZZs in the name of the file. Here is the code : StringReplace[DateString[], h : (_ ~~ _) ~~ ":" ~~ m : (_ ~~ _) ~~ ":" ~~ s : (_ ~~ _) :> h ~~ "h" ~~ m ~~ "m" ~~ s ~~ "s"]. It solves a second problem too : sometimes (depending on the operating system) the seconds have decimals (10.45 for instance) $\endgroup$
    – andre314
    Commented Jul 8, 2019 at 18:45
  • 1
    $\begingroup$ If you really want to put colons in file names, I suggest you use Linux. However, even there, I think they are better avoided. $\endgroup$
    – mikado
    Commented Jul 8, 2019 at 19:24
  • $\begingroup$ Using the StringReplace info from @andre314, you might also consider using DateString["ISODateTime"] such as this: StringReplace[DateString["ISODateTime"], h : (_ ~~ ) ~~ ":" ~~ m : ( ~~ ) ~~ ":" ~~ s : ( ~~ _) :> h ~~ "h" ~~ m ~~ "m" ~~ s ~~ "s"] The result is a more compact representation. $\endgroup$
    – Mark R
    Commented Jul 9, 2019 at 0:45

1 Answer 1


This is not possible. A colon is not a valid character in a file name on Windows.

Reference: https://docs.microsoft.com/en-us/windows/win32/msi/filename

  • $\begingroup$ You're right @Szabolcs, thanks! I am just so used to Linux. $\endgroup$ Commented Jul 8, 2019 at 20:18
  • $\begingroup$ Even if valid, it would be prone to all sort of problems. I would avoid spaces, most symbols and long strings in filenames as much as possible. $\endgroup$
    – rhermans
    Commented Jul 9, 2019 at 13:57

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