Adding time values into video or as subtitles

[Heliooos]

Hi all,
I have videos from some research experiment - some are recorded with standard HD or SD PAL cameras, one is hi-speed (300fps) video from Casio Exilim F1 (I deleted some additional info which are not important):

Format                           : MPEG-4
Format profile                   : QuickTime
Codec ID                         : qt  
Overall bit rate                 : 1 392 Kbps
Writing application              : Casio Digital Camera

Video
ID                               : 1
Format                           : AVC
Format/Info                      : Advanced Video Codec
Format profile                   : Main@L4.0
Format settings, CABAC           : Yes
Format settings, ReFrames        : 1 frame
Codec ID                         : avc1
Codec ID/Info                    : Advanced Video Coding
Duration                         : 45mn 18s
Bit rate                         : 1 390 Kbps
Width                            : 512pixels
Height                           : 384pixels
Display aspect ratio             : 4:3
Frame rate                       : 29,970 fps
Resolution                       : 8 bits
Colorimetry                      : 4:2:0
Scan type                        : Progressive
Bits/(Pixel*Frame)               : 0.236
Stream size                      : 450 MiB (100%)
Language                         : English

I would like to add a time values in the video or as external subtitles to show ho fast the recorded process is (cannot say more info about our research). I am using Avidemux + some additional tools like Avisynth, Avanti etc. and we also have one Pinnacle Studio 12 (the basic version).

Is there any free tool which could do it? The best way would be, if I could also somehow "calibrate" the hi-speed video to show approx. real time in the titles.

any advice would be nice

thanks

[Gavino]

Avisynth has a ShowTime function.