We can look at this in terms of a rate. . .
1m : 1000 years ...... Divide both sides by 1000
0.001 m : 1 year ........ change units
1 mm : 365 days ......... divide by 365
0.0027397mm (roughly) : 1 day ........ * by 2.5
0.0068493 mm (roughly) : 2.5 days
(corrected)
