the declining sun in the west
昳 centers on the sun declining westward, capturing the moment when daylight fades and the sun sinks toward the horizon.
昳 combines 日 (sun) with 失, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is associated with the sun declining.
The sun 日 on the left is losing its height, like something 失 (lost) in the sky. Picture the sun slipping away as it declines in the west.
For テツ, imagine the sun setting and a metal tetsu (鉄, iron) bell tolling to mark the end of the day: tetsu -> テツ, as the sun declines.