The current specification says that the MTIME values seen by two HARTs to be not apart by more than 1. It does say the MTIME must always increment in units of 1.  I do not think the specification mandates incrementing by one on each clock tick. Presently it says the tick - the update frequency - can be 100 MHz or 10 MHz or somewhere in between. If the update frequency is 10 MHz then the MTIME increment per clock must be 10. If the update frequency is 100 Mhz then MTIME increment per clock is 1. So is your concern is that an adder that adds 10 to MTIME per clock tick is hard?