With PhotoMechanic, this is not exactly possible as you describe it. What you can do is adjust the datetime by a certain "delta". This actually works great if your camera clock was set incorrectly (for instance you forgot to adjust to daylight savings time, or a change of timezone).
Select the series of images you want to adjust, then under tools you find the menu entry "Adjust Capture Dates and Times...", select that and now either set the "delta" using the "relative" adjustment, or set the correct datetime of the first image using "absolute" adjustment (in this case PM calculates the delta and adjusts the other selected images accordingly). If you need more help, this is all described in the manual as well, or just come back and ask more questions
Cheers,
Hayo