In my grammer book there is a sentence:
I always stay on duty (since/untill) six o'clock.
And you should choose between "since" and "untill". I can't understand, why it is "untill". I thought that it's no matter in fact what to use "since" or "untill" in this case.
In that context, "until" means, up to or as late as, whereas "since" means, from then to now. "until" is the correct choice.
Here are a few examples for "since":
EX: I been working here since Friday. (throughout a period)
EX: What have you been doing since Friday? (the time after Friday)
EX: I haven't seen Max since Friday. (from that time until now)