This is the number of days the two witnesses will preach. It is an odd amount, so let's have a brief look at these times of 3.5 years, 1260, 1290, 1335 days, and 42 months. Are these Gregorian or Jewish periods
The Gregorian calendar year is based on a solar cycle, but the Jewish year is based on lunar cycles. A lunar cycle lasts 29.5 days, which is rounded up to 30 to make the 360 days most often mentioned. This is five days short of a solar year. If there were no adjustments made each year, mid-winter would move to mid-summer every 36 years. This is adjusted for when the days in the months change. Some Jewish years have ended up with over 380 days.
3.5 years in Gregorian is 1278 days; Jewish days are 1260. Therefore, their mission lasts for about the same time as the Antichrist's. They will arrive on the scene just before he is revealed. It finishes just before Christ's return.
When you see 3.5 years, that is a solar and current measurement. Months will be solar, and days are just days, but can have Jewish lunar significance. What the 1290 days and 1335 days are is not so clear. Those days may relate to the conflict at the end of the 1000 years. Just a note about the Julian Calendar. The Julian calendar is a solar calendar that used 365.25 days in a year. As measurements became more accurate, we changed to the Gregorian calendar, which is 365.13 days. Since the time of Christ, there is an 11-day difference. Not a big difference.