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(Pocket-lint) - There's yet another hint the iPhone 5S will be launched by Apple on 20 September. T-Mobile US has blacked-out dates between 20 and 22 September for employees, meaning they're unable to take vacation time during the period. TmoNews was the first to uncover the tidbit and hears from sources that it could be iPhone related.

It's substantial that employees won't be able to take vacation time on 20 September and the following weekend, because T-Mobile will need the extra help in its stores to sell the new iPhone 5S and iPhone 5C. We've seen this for several iPhone launches in the past, where Apple and carrier retail employees are unable to take off so all hands are on deck. 

The 20 September launch date doesn't sound too far-fetched either. Apple typically holds a press event on a Tuesday or Wednesday and launches the iPhone a week and a half later on a Friday. We saw this with the iPhone 5 and iPhone 4S.

Apple has still to confirm the 10 September press event it's planning to hold, though Pocket-lint and several other publications have it from sources. The company is expected to announce the iPhone 5S and iPhone 5C.

Given the iPhone 5S naming, we can already expect speed bumps. Noted KGI analyst Ming-Chi Kuo predicts Apple will launch the A7 system-on chip design with 1GB of LPDDR3 RAM with the handset, said to boost speeds by up to 20 per cent. Furthermore, the camera will feature a 8-megapixel camera like the iPhone 5, though it will use a larger f2.0 aperture lens assembly with dual LED flashes, as we've heard in previous rumours. A fingerprint sensor in the home button has also been tipped.

As for the iPhone 5C, it is said to be made out of plastic and aimed at emerging markets and customers on a budget - with a colourful flavour added.

The Wall Street Journal reported this month that Apple supplier Hon Hai/Foxconn has been asked to ship the two new iPhone models it's been building for Apple in "early September". Wednesday's T-Mobile news gels nicely with the earlier report.

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Writing by Jake Smith. Originally published on 28 August 2013.