This tool is extract from Adafruit_GFX for generating font files for Arduino.
You may see more details at Adafruit Learning System: https://learn.adafruit.com/adafruit-gfx-graphics-library/using-fonts
The font files include the characters from ASCII code 32(Space) to 126(~)
The font files only inculded 11 charaters, ASCII code 48-57(number 0-9) and followed ASCII code 58(:). This can much reduce required program size if only want to use that font for displaying clock time.