Update README.md

Added file name argument, structure of save file and build information.

README.md

@@ -15,11 +15,29 @@ TimeSave consists of two programs:
 It's a good idea to start TIMEREST.COM in your AUTOEXEC.BAT before anything that needs current timestamps.
 TIMESAVE.COM can be started at any time, like at the end of a batch file after you've exited Windows.
 
+As of v20240519, an alternative file name can be passed as a sole program argument.
+Files created in this way are not marked hidden, unlike TIMESAVE.DAT in its default location.
+
+## Structure of TIMESAVE.DAT
+
+The save file consists of 6 little-endian words, resulting in a total of 12 bytes.
+
+| Offset | Value                    | Remarks
+|-------:|--------------------------|-------------------------------------------------
+|      0 | Signature                | Always 0x5354 (`TS`)
+|      2 | Checksum                 | all words including the checksum total to zero
+|      4 | Year                     | full four-digit year, not 1980-based
+|      6 | Month and day            | as passed in DX register of DOS date functions
+|      8 | Hour and minute          | as passed in CX register of DOS time functions
+|     10 | Seconds and centiseconds | as passed in DX register of DOS time functions
+
 ## Building
 
-TimeSave is written using Turbo Assembler. A suitable makefile is supplied. Type `make` to build the programs,
+TimeSave is written using Borland Turbo Assembler 2.0. A suitable makefile is supplied. Type `make` to build the programs,
 or `make -DDEBUG` to assemble and link with debug information suitable for Turbo Debugger.
 
+It can be assembled using contemporary versions of Turbo Assembler, but debug information is not retained in COM programs during linking.
+
 ## Screenshot
 
 ![scrshot](https://github.com/DosAmp/timesave/assets/592891/73d54c76-0644-40f5-b653-384af2e87d3d)