If you type a numeric literal with a physical unit, there should be a space between the number and the unit.
T := 25ns; -- ILLEGAL, but accepted by ModelSim T := 25 ns; -- OK; according to VHDL language standard
Mentor Graphics’ ModelSim and QuestaSim accept the former (illegal) version. As a result, some VHDL designers got used to writing the incorrect version, producing code that is not portable to other simulators. Sigasi Studio accepts the ModelSim-style physical literals but warns about this (rule 47).