The '--annotate' (-a) flag allows data (time/date etc) text annotations to the image. By default, you get black text on a white background
The data is selected by setting it's 'bit'. For example, for time (b2 = 4) and date (b3 = 8) use '-a 12'. You can add your own text may include date/time placeholders by using '%' character as used by strftime. Value Meaning Example Output b0 b1 b2 (4) Time 20:09:33 b3 (8) Date 10/28/15 b4 (16) Shutter Settings b5 (32) CAF Settings b6 (64) Gain Settings b7 (128) Lens Settings b8 (256) Motion Settings b9 (512) Frame Number b10 (1024) Switch to white text on black background Examples:- -a "ABC %Y-%m-%d %X" Show some text ABC %Y-%m-%d %X -a 4 -a "ABC %Y-%m-%d %X" Show custom formatted date/time. ABC 2015-10-28 20:09:33 -a 8 -a "ABC %Y-%m-%d %X" Show custom formatted date/time. ABC 2015-10-28 20:09:33 -ae Set extra annotation parameters Specifies annotation size, text-colour, background-colour. Colours are in hex YUV format. Size ranges from 6 to 160, default is 32. Asking for an invalid size should give you the default. Example Explanation -ae 32,0xff,0x808000 -a "Wibble gibber gibber" gives size 32 white text on black background -ae 10,0x00,0x8080FF -a "Wibble gibber gibber" gives size 10 black text on white background
Above information copied from Raspberry Pi camera documentation