• GetTextAsDate ("07.19.01" ; "yy.dd.mm")
• GetTextAsDate ("Jan 19, 2007" ; "mmm dd, yyyy")
• GetTextAsDate ("20080616"; "yyyymmdd")
• GetTextAsDate ("Friday, Jan 19, 2007" ; "day, month dd, yy")

These all equate to a FileMaker date output of 1/19/2007. Read the comments at the bottom of function for some limitations and non-obvious behavior.

If you’re new to FileMaker development, you’ll need to know that FileMaker Advanced is required to create, but not use, Custom Functions. 

// G e t T e x t A s D a t e
// 
// Function Parameters:
//    dateTxt    raw date text to convert into FMP date
//    formatTxt  string indicating the format of the date

Let (

[
  // Clean up a bit to be on safe side.
  dateTrimmed = Trim (dateTxt);

  // These two date formats have synomyms. We do the substitutions here to simplify the case logic.
  format = Substitute (Substitute (formatTxt; "yyyy"; "yy"); "month"; "mmm");

  // Make date string into something we can parse elements out of by replacing "-", ".", and "/" with a space.
  dateWords = Substitute (dateTrimmed; ["-"; " "]; ["/"; " "]; ["."; " "])
 
];

// Each date format will use a different expression in the case statement.

Case (

  // The IF block returns true if this is a format we want to interpret.

  If (format="dd-mmm-yy" or formatTxt="dd/mmm/yy" or formatTxt="dd.mmm.yy"; 1; 0);
     // In this instance, the month is encoded as a letter abbreviation or its full name, so we must convert the month string to its ordinal.
     // We also use the FMP Date function to convert the year seperately so that we can rely on FMP to convert 2-digit years appropriately.
     Date (GetMonthAsNumber (MiddleWords (dateWords; 2; 1)); LeftWords (dateWords; 1); Year (GetAsDate ("1/1/" & RightWords (dateWords; 1))));

  If (format="dd-mm-yy" or formatTxt="dd/mm/yy" or formatTxt="dd.mm.yy"; 1; 0);
     Date (MiddleWords (dateWords; 2; 1); LeftWords (dateWords; 1); Year (GetAsDate ("1/1/" & RightWords (dateWords; 1))));

  If (format="mm-dd-yy" or formatTxt="mm/dd/yy" or formatTxt="mm.dd.yy"; 1; 0);
     Date (LeftWords (dateWords; 1); MiddleWords (dateWords; 2; 1); Year (GetAsDate ("1/1/" & RightWords (dateWords; 1))));

  If (format="yy-mm-dd" or formatTxt="yy/mm/dd" or formatTxt="yy.mm.dd"; 1; 0);
     Date (MiddleWords (dateWords; 2; 1); RightWords (dateWords; 1); Year (GetAsDate ("1/1/" & LeftWords (dateWords; 1))));

  If (format="yy-dd-mm" or formatTxt="yy/dd/mm" or formatTxt="yy.dd.mm"; 1; 0);
     Date (RightWords (dateWords; 1); MiddleWords (dateWords; 2; 1); Year (GetAsDate ("1/1/" & LeftWords (dateWords; 1))));

  If (format="mmm dd, yy" or format="month dd, yy"; 1; 0);
     Date (GetMonthAsNumber (LeftWords (dateWords; 1)); MiddleWords (dateWords; 2; 1); Year (GetAsDate ("1/1/" & RightWords (dateWords; 1))));

  If (format = "day, month dd, yy" or format = "day, mmm dd, yy"; 1; 0);
     // We ignore the day string, and instead rely on dd.
     Date (GetMonthAsNumber (MiddleWords (dateWords; 2; 1)); MiddleWords (dateWords; 3; 1); Year (GetAsDate ("1/1/" & MiddleWords (dateWords; 4; 1))));

  format="yymmdd";
     // This format is actually always a 4-digit year (yyyy), but the earlier format subsitution clobbered the format.
     Date (Middle (dateTrimmed; 5; 2); Right (dateTrimmed; 2); Left (dateTrimmed; 4));

  // DEFAULT
  // We can't evaluate the format given.

  "invalid format: " & format
 )
)


/* ----------------------------------------

 # Function:        Convert a text string of given format into a date value.
                    Returns an error message as its result if an invalid format string was used. 2-digit years follow FileMaker's convention for
                    conversion to 4-digit years. Values are NOT range checked, but this does mean you can use, eg, 13/1/2007 to get 1/1/2008.

 # Parameters:      formatTxt:

                    This is the date format that will be used with input string. For all of these, the year
                    can be in either 2 or 4 digit format. The available formats are below, grouped by their related synomyms:

                            day, mmm dd, yy     Note: the day (eg "Tuesday") is ignored. You can safely pass a timestamp string (everything after year is ignored).
                            day, mmm dd, yyyy
                            day, month dd, yy
                            day, month dd, yyyy

                            dd-mm-yy
                            dd/mm/yy
                            dd.mm.yy
                            dd-mm-yyyy
                            dd/mm/yyyy
                            dd.mm.yyyy

                            dd-mmm-yy            Note: mmm = 3 char month abbreviation (ie, JAN), not a number
                            dd/mmm/yy
                            dd.mmm.yy
                            dd-mmm-yyyy
                            dd/mmm/yyyy
                            dd.mmm.yyyy

                            dd-month-yy           Note: month = month name (ie, january), not a number
                            dd/month/yy
                            dd.month.yy
                            dd-month-yyyy
                            dd/month/yyyy
                            dd.month.yyyy

                            mm-dd-yy
                            mm/dd/yy
                            mm.dd.yy
                            mm-dd-yyyy
                            mm/dd/yyyy
                            mm.dd.yyyy

                            mmm dd, yy             Note: mmm = month's 3 letter abbreviation, month=month's full name
                            mmm dd, yyyy
                            month dd, yy
                            month dd, yyyy

                            yy-mm-dd
                            yy/mm/dd
                            yy.mm.dd
                            yyyy-mm-dd
                            yyyy/mm/dd
                            yyyy.mm.dd

                            yyyymmdd               Note: the month & day must be zero filled
                    dateTxt: 
                            The date string to be converted

 # Created:         2008-Dec-15 10h52  simon_b beezwax.net
 # Modified:        2009-Jan-14 13h34  simon_b beezwax.net  converted nested substitutes to list form
 # Author:          Simon Brown
 # Requires:        GetMonthAsNumber custom function
<---------------------------------------- */ 

The GetTextAsDate function requires a helper function, GetMonthAsNumber. This is a trivial CF used to convert an English month name into its ordinal value:

GetMonthAsNumber (monthTxt)

Case (
  monthTxt = "jan"; 1;
  monthTxt = "January"; 1;
  monthTxt = "feb"; 2;
  monthTxt = "February"; 2;
  monthTxt = "mar"; 3;
  monthTxt = "March"; 3;
  monthTxt = "apr"; 4;
  monthTxt = "April"; 4;
  monthTxt = "may"; 5;
  monthTxt = "jun"; 6;
  monthTxt = "June"; 6;
  monthTxt = "jul"; 7;
  monthTxt = "July"; 7;
  monthTxt = "aug"; 8;
  monthTxt = "August"; 8;
  monthTxt = "sep"; 9;
  monthTxt = "September"; 9;
  monthTxt = "oct"; 10;
  monthTxt = "October"; 10;
  monthTxt = "nov"; 11;
  monthTxt = "November"; 11;
  monthTxt = "dec"; 12;
  monthTxt = "december"; 12;

  // default
  0
)

/* ----------------------------------------

 # Function:         Convert a month string, either full name or 3 char abbreviation, into its ordinal value (base 1)
 # Parameters:       monthTxt: String to be converted
 # Created:          2008-Jan-15 simon_b@beezwax.net
 # Modified:         
 # Author:           Simon Brown

---------------------------------------- */

 I hope this helps with your conversion of dates!