truly, wholly, cordial
亶 centers on sincerity and completeness: being truly, wholly, or cordially something. The meanings share an idea of genuine, unreserved quality.
亶 combines 㐭 (a granary) with 旦 (dawn). The exact historical development is uncertain, but it likely expressed abundance or fullness, leading to the idea of sincerity and completeness.
The top 㐭 looks like a full granary, and the bottom 旦 is the rising sun. Picture a granary filled to the brim at dawn, representing true abundance and wholehearted generosity.
For タン, imagine a tan-colored granary full of grain, truly and wholly packed: tan -> タン, and the granary's fullness shows sincerity.