except that the clock is integrated in the MSX system chip from S1985
only MSX2s with an S3527 have an RP5C01 clock in chip form https://www.msx.org/wiki/Yamaha_S3527
I think Grauw is absolutely correct in his response... MSX2 and on must have the RTC.
Of course there are non-standard versions, like some conversions or consolized versions of the hardware that doesn't have it, but those are not standard and not prevalent.
So it is valid to think that counting on having the RTC might lower a little bit compatibility with those outliers, but that doesn't change the answer: any MSX 2 or better that follows the standard has the RTC.
