Is it possible to make characters in Flutter to have the same width?

You can set the font feature to use tabularFigures.

// import 'dart:ui';

Text(
  _getTimeString(),
  style: TextStyle(
    fontFeatures: [
      FontFeature.tabularFigures()
    ],
  ),
),

Before:

After:

See also:

• Font Features in Flutter

Use a monospaced font, also called a fixed-pitch, fixed-width, or non-proportional font, is a font whose letters and characters each occupy the same amount of horizontal space.

Wikipedia explains it well. https://en.wikipedia.org/wiki/Monospaced_font

Hiepav suggestion, seems a good approach because you are not doing nothing wrong but each character in the font have different widths so it will have to adjust to give enough room.

However, as a workaround you can actually wrap your texts in a fixed width sized box that gives enough space for both widgets regarding its character widths variations , such as SizedBox, ConstrainedBox or even Container with width constraints and center align child. This way, with that font, you should at least have the : vertically aligned.