The difference is essentially to do with a focus on the end-point of the period in question. The normal usage is since, as the 'have been' verb form focuses the attention on the beginning of the time period, and says that this time is currently relevant. From focuses the attention on the end-point of the time period (as in 'from X until Y'), and this clashes with current relevance, as that time period is over. We would normally see from used with other verb forms, therefore (I am/will be in London from Monday to Friday or I stayed there from January to March).
The operative word is 'normally', as sentences like the one used by my correspondent can certainly be heard by native speakers. What's happening is that they are blending two constructions - something that happens a lot in spontaneous speech. There's been a change in mental focus while the sentence is being said. A similar switch explains 'I've seen him a week ago'. I talk more about blending in the paper I gave to IATEFL last year - downloadable from my website (go to Books and Articles and type 'blends' into the search box).
[Footnote, for those who have noticed: this is a post after a relatively long period of bloglessness, due to various book projects coming to the boil at the same time. The first of these, Words in Time and Place, is an introduction to the historical thesaurus of the OED, and will be published by OUP in September. As with any lexicographical project, the grind of working through words, whether semantically or alphabetically, leaves little time for much else. Surfacing at the end of Z is a bit like coming out of social hibernation.]