This is actually a very obscure English–Latin pun.
The Latin for 'now' is nunc; that's if you're speaking (or writing) about the actual present moment: as in "I'm doing it now".
Another sense of the English word 'now' is synonymous with 'any more' – for example: "No one does that now". A third sense is synonymous with 'already': "We are doing that already" means we're doing it now, as opposed to at some point in the future.
In either of these latter two senses, the Latin word would be iam, as opposed to nunc.
Wikipedia explains this as follows: iam is the Latin word for 'now', but only in the past or future tense. In the present tense, it's nunc. So it's iam if you're talking about yesterday, iam if you're talking about tomorrow, but nunc if you're talking about today.
Note that there is no J in Latin, so 'jam' would be spelt iam.
© Haydn Thompson 2017