It is not a bad sentence unless you take it ONLY to mean physically fit. It sounds like the consensus is that is also can mean well or just not up to one's usual standards. For example, if they were ill or having trouble focusing due to ourside stressors. But we still don't have a native to verify these other uses of the word.
For native German speakers: "Today I am not in shape" is ambiguous. "In shape" means physically fit. It is not something that changes on a daily basis. Also, today is more commonly used to mean this calendar day, otherwise we say "Nowadays" or "These days" to talk about the present. For performance in work or sport It would be more natural to say:
Today I am not on form.
Today I am not in form.
For temporary physical conditions or mood it would be more natural to say.
Today I am not in good form.
However, If you want to say that you used to be physically fit, and you no longer are, it would be clearer to say
Nowadays I am not in shape
These days I am not in shape
I am not in shape now
But what does "Heute bin ich nicht in Form." Mean in German? Does it mean your performance on a given day? Or does it mean your physical fitness at this point in your life?
Yes, of course. I have heard that too. But "I'm not in good shape today" is another example of an ambiguous sentence.
I was drinking last night. I'm not in good shape today.
I was an athlete in high school. I'm not in good shape today.
So you see how the sentence is ambiguous, without context. If a native German speaker is aware that there is ambiguity they could choose different words to more clearly express what they mean. (The examples I have given in the comment above).
As both "in form" and "on form" are used in both English and American English, they will both appear in dictionaries however; are you sure both versions mean exactly the same thing? "In form" is correct when talking about your level of fitness or to put it another way, when using 'form' as a synonym for 'shape' (in form and colour, I'm in form right now - less common) but when referring to "performance", the correct (or 'most common') expression is: "to be on form". Consider:
I was on point today
I was on my best behaviour today
I was on fire today (slightly different grammar, I know!)
I was on the ball today
I'm on it
I've been on form all day
I'm not saying there aren't people who use 'in' when referring to their performance, but it is uncommon and better to learn "on form".
In Australia it's much more common to use "in form". The term "based on form" is usually reserved for discussions about the recent performance of horses or sports people. The following sentences would be correct :
"The horse will not win based on form".
No comments about "based off form" please :-)
"He will do well if he is in form today."
I'm not sure anyone mentioned "based" but that's definitely another good example.
"In form" is correct when talking about your level of fitness or to put it another way, when using 'form' as a synonym for 'shape' - Yes, I agree with your final sentence. My comment discussed describing performance rather than describing physical fitness or a general state of readiness.
yes and no.
Heute bin ich nicht in Form. - is right.
Mit dem heutigen Tag sind es fünfzehn Jahre. ~ I my preferred suggestion.
Seit heute sind es fünfzehn Jahre. / (Heute sind es 15 Jahre.)
The word "es" is necessary, because you need a subject for the sentence.
- Es sind mit dem heutigen Tag 15 Jahre.
- Es sind seit heute 15 Jahre.
- (Es sind heute 15 Jahre.)