Your phone or tablet must have the date/time and timezone settings set up correctly. Make sure your device reflects the actual Daylight Saving Time setting effective in where you stay.
This app needs to know your estimated location to function. GPS accuracy is not necessary.
Internet access is required for a variety of features, including but not limited to: address to coordinates translation, timezone, and DST checks, accessing the online profile database, etc...
Misc features may need and ask for additional permissions.
A valid and up-to-date Google Play Services which is allowed to run in the background. If you don't have Google Play Store installed on your device, you may get it from here.
A Unicode 6+ font set as system default to display various symbols correctly. You can install custom fonts if you have root access, or you can use the menu provided by your device manufacturer to select another. If you are having trouble replacing the existing font, you may try replacing your Launcher; they usually offer various font choices.
Android 4.4+ is required, 7+ is highly recommended, 9+ is great. The app will behave differently depending on the Android version.
The minimum supported device-width in DP is 320; screen elements may not fit otherwise (rotating the device may help).
At least 1GB system memory to run smoothly, 2GB+ highly recommended.
Make sure you are not blocking internet access in any way; try changing DNS settings if you suspect this might be the case.
Make sure you use the same Google Play account for all the purchases.
Try not to skip versions between updates to avoid compatibility issues. Otherwise, you may need to clear the app data/cache and reinstall the app again.
In addition to these, please note that,
Arctic regions where the concepts of day and night start to disappear (≈65+ latitude) are not supported.
If you are residing in China, you will need a VPN service to get through The Great Firewall.