Create an Arduino compatible clock inexpensively. This project is fun and easy to duplicate. It can be put into the enclosure or project of choice. I used a plastic electronics parts kit box. The base parts cost $5, but it also needs micro USB power. The sketch coded for 24 hour time.

Step 1: Parts

Part List - Links to AliExpress

Step 2: Digispark Support

Digispark needs a USB driver installed. Instructions for Windows 7 - 10

Add Arduino IDE board support for Digispark

Go to the "File" menu and select "Preferences"
In the box labeled “Additional Boards Manager URLs” enter: http://digistump.com/package_digistump_index.json and click OK

Go to the “Tools” menu and then the “Board” select “Boards Manager” and then from the type drop down select “Contributed”: Select the “Digistump AVR Boards” package and click the “Install” button.

After install completes, close the “Boards Manager” window and select “Digispark (Default - 16.5mhz)” from the Tools→Boards menu.

Step 3: Program With Arduino IDE

Go to Sketch, Include Library, then Manage Libraries. Make sure the following libraries are installed: tm1637 (Grove 4-Digit Display)

Open the sketch file and set the correct time on the rtc.adjust line. The numbers are: (Year, Month, Day, Hour, Minute, Second)

These Digispark style development boards work differently than Arduino boards. You will first hit upload then plug in the board when prompted. They are programmable for a few seconds after plugged in.

5clock.zip
Download

Step 4: Assemble Parts

You will need to do some light soldering to install the pin headers onto the modules.

Put CR2032 battery in in DS3231 RTC module
Connect jumper wire from P0 to SDA on the RTC module
Then connect P2 to SCL on RTC module
Connect P3 to CLK on TM1637 Display module
Then P4 to DIO on Display module
Connect VCC and Ground to RTC module then VCC and Ground on other side to the display module.

All done! You can power it now. You can use a micro USB AC adapter or battery pack.