Michael was born on 8 November 1942, the night the bells rang for the Second Battle of El Alamein. On 29 October 1962, the last day of the Cuban Missile Crisis, he would have been just over a week from his 20th birthday. However he is shown as still being at school doing his A levels (usually done at age 17 or 18).
When the bloke in the phone shop asks Gary for Ron's mobile number, he should have already known that mobile phone numbers started with 07. The 07 dialling code came in well before 1999, Gary and Yvonne both had mobile phones, so he should have known this.
If Gary was born in 1962, he would have been 37 when the time portal closed in 1999 and he was stuck in 1945. Adding another 17 years, he would have been 54 rather than 52.