A datetime value consisting of monotonically increasing integer units measured from a specific moment in time

This is correct.

since there are 86,400 seconds in a day and 1000 ms in a second.

Here it gets confusing. That's only true for some definitions of days. UTC days, for example, aren't always 86 400 seconds long.

Nit: Use a (thin) space character for grouping digits. BIPM and IUPAC recommend it.
https://en.wikipedia.org/wiki/Decimal_separator#Digit_grouping
https://en.wikipedia.org/wiki/Decimal_separator#Unicode_characters

Universal Coordinated Time or UTC is the basis for modern timekeeping. Among other things, it provides a common baseline for converting between incremental and wall time.

And here it's just blatantly incorrect. Incremental time is used more often on top of UTC (Unix time) than below it (TAI), but it's not intrinsically on either side. You can't say that UTC can convert between any incremental and wall time. Furthermore, wall time is (debatably; I believe what you're referring to has traits of wall time, floating time, and field-based time) only defined by a calendar like ISO 8601, so wall times shouldn't matter to UTC.

Did you mean the conversion to be like converting from UTC+0 to local time? I guess it's natural to write it from that point of view as this project has a major focus on time zones.

UTC should also be described as being a count of SI seconds, based on TAI, but that adds/removes (terminology?) leap seconds to stay in sync of UT1, a time standard following mean solar time.