Storage

The SPI ePaper screens does not include any fonts and need to generated by code. To make a graphical rich display you need lots of storage. 

Three solutions 

1. On board Flash of your MCU.
2. SD card.
3. Flash chip.

The on board flash  of MCU chips is normally up to 512Kbytes. The high end with more Flash are normally big chips and high powerful chips that use power.

SD cards have lots of merit. Easy to get and work with. But is power hungry and needs 3.3 V. Can't use a coin cell with out a boost converter. For a clock that updates every minute not good.

I'm using SPI Flash chip. They use not mush power and can run to 1.8V/2V  The are uselessly in bits, so the 32Mbits  I use is 4Mbytes. Can hold lots of different font styles

Format and layout

Each pixel is a bit. The internal resolution of the 2.13 display is 250 x 128 , so to fill up the screen will take  4000 bytes

Clock fonts

My clock works with a left side

and a right side

For a given clock font 24 x 2 (24 / am pm) images left . And 60 for minutes for right  .