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Subclause 6.2.5, paragraph 911, of the C Standard [ISO/IEC 9899:20112024] provides the necessary guarantees to ensure this solution works on a conforming implementation:

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Conversions to signed character types are more problematic.

The C Standard, subclause 6.3.1.3, paragraph 3 [ISO/IEC 9899:20112024], says, regarding conversions

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Furthermore, subclause 6.2.6.2, paragraph 2, says, regarding integer modifications

If the sign Each bit is one, the value shall be modified in one of the following ways:
— the corresponding value with sign bit 0 is negated (sign and magnitude)
— the sign bit has the value −(2M ) (two’s complement);
— the sign bit has the value −(2M − 1) (ones’ complement).
Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all value bits zero (for the first two), or with sign bit and all value bits 1 (for ones’ complement), is a trap representation or a normal value. [See note.]

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that is a value bit shall have the same value as the same bit in the object representation of the corresponding unsigned type. If the sign bit is zero, it shall not affect the resulting value. If the sign bit is one, it has value −(2N−1). There may or may not be any padding bits signed char shall not have any padding bits. 

Consequently, the standard allows for this code to trap:

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Implementations with such trap representations are thought to have existed in the past. Your author was unable to locate any documents describing such processors.

INT31-C-EX3: ISO The C Standard, section subclause 7.2729.2.45, paragraph 3 [ISO/IEC 9899:2024] says:

The time function returns the implementation’s best approximation to the current calendar time. The value (time_t)(−1-1) is returned if the calendar time is not available.

If time_t is an unsigned type, then the expression ((time_t) (-1)) is guaranteed to yield a large positive value.

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