Awhile back we developed a DMM & Scope Device just to "check" our various instruments.
https://www.eevblog.com/forum/testgear/ac-rms-dmm-tests/Note the last couple posts where we had to replace the Voltage Reference which died for some unknown reason. Likely we shorted one of the exposed banana plugs to something, as we are not exactly "gentle" and it gets tossed around the lab, but do try and keep it powered 24/7.
This device has been used to "verify" handhelds Fluke 77 and 87, U1233A & UT210E. Also used for lab bench 6 1/2 digit DMMs HP34401A, AG34401A, SDM3065X, three KS34465A (light and 2 dark) and a Keithley DMM6500 as well as our DSOs.
We were curious as to how the old analog IC RMS converters behaved verses the Digital RMS Computational Methods in the KS34465A and DMM6500. So comparing the 34401A vs. the 34465A and DMM6500 reveled the analog IC RMS techniques held up very well indeed

Of course YMMV.
Edit: Just did quick "Turn on and Read" each instrument, no proper warm up and the lab temp is ~24C and humidity ~58% (normally ~26C & 45%). OK need to warm up the lab it's cold here (~15C), so justifies turning on all these instruments

Handhelds:
35 year old Fluke 87 2.502VAC (never calibrated)
KS U1233A 2.507VAC
UT210E 2.45VAC
Bench: Analog RMS Converters
HP34401A 2.50005VAC
AG34401A 2.49978VAC
SDM6500 2.49723VAC
Bench: Computational RMS
DMM6500 2.499945VAC
KS34465A-1 2.5000405VAC
KS34465A-2 2.4998607VAC
KS34465A-3 2.4999653VAC
So think one can surmise that this square-wave technique described in the link above is acceptable to quickly verify typical DMMs used in repair and around the lab with AC RMS, and are working properly, or not!!
Read is NOT a claim to ultimate precision, altho one would have difficulty arguing against this technique and results, but simply an easy, low cost, simple, means to VERIFY a DMMs proper AC operation within the constraints of the squarewave stimulus.
Best & Happy Holidays,